diff --git a/constructorio-client/src/main/java/io/constructor/client/ConstructorIO.java b/constructorio-client/src/main/java/io/constructor/client/ConstructorIO.java
index 0ac7968a..c60ad280 100644
--- a/constructorio-client/src/main/java/io/constructor/client/ConstructorIO.java
+++ b/constructorio-client/src/main/java/io/constructor/client/ConstructorIO.java
@@ -2031,6 +2031,50 @@ protected static String getResponseBody(Response response) throws ConstructorExc
         throw new ConstructorException(errorMessage, errorCode);
     }
 
+    /**
+     * Validates and extracts the file extension from a File object for catalog uploads.
+     * Only .csv, .json, and .jsonl extensions are supported.
+     *
+     * @param file the File object containing the actual file
+     * @param fileName the logical file name (items, variations, item_groups)
+     * @return the validated file extension (including the dot, e.g., ".csv", ".json", or ".jsonl")
+     * @throws ConstructorException if the file extension is invalid or missing
+     */
+    private static String getValidatedFileExtension(File file, String fileName)
+            throws ConstructorException {
+        if (file == null) {
+            throw new ConstructorException(
+                    "Invalid file for '" + fileName + "': file cannot be null.");
+        }
+
+        String actualFileName = file.getName();
+        if (actualFileName == null || actualFileName.isEmpty()) {
+            throw new ConstructorException(
+                    "Invalid file for '" + fileName + "': file name cannot be empty.");
+        }
+
+        int lastDotIndex = actualFileName.lastIndexOf('.');
+        if (lastDotIndex == -1 || lastDotIndex == actualFileName.length() - 1) {
+            throw new ConstructorException(
+                    "Invalid file for '"
+                            + fileName
+                            + "': file must have .csv, .json, or .jsonl extension. Found: "
+                            + actualFileName);
+        }
+
+        String extension = actualFileName.substring(lastDotIndex).toLowerCase();
+
+        if (!extension.equals(".csv") && !extension.equals(".json") && !extension.equals(".jsonl")) {
+            throw new ConstructorException(
+                    "Invalid file type for '"
+                            + fileName
+                            + "': file must have .csv, .json, or .jsonl extension. Found: "
+                            + actualFileName);
+        }
+
+        return extension;
+    }
+
     /**
      * Grabs the version number (hard coded ATM)
      *
@@ -2308,9 +2352,12 @@ protected static JSONArray transformItemsAPIV2Response(JSONArray results) {
     /**
      * Send a full catalog to replace the current one (sync)
      *
-     * @param req the catalog request
-     * @return a string of JSON
-     * @throws ConstructorException if the request is invalid.
+     * Supports CSV, JSON, and JSONL file formats. The file type is automatically
+     * detected from the file extension (.csv, .json, or .jsonl).
+     *
+     * @param req the catalog request containing files with .csv, .json, or .jsonl extensions
+     * @return a string of JSON containing task information
+     * @throws ConstructorException if the request is invalid or file extensions are not supported
      */
     public String replaceCatalog(CatalogRequest req) throws ConstructorException {
         try {
@@ -2336,10 +2383,11 @@ public String replaceCatalog(CatalogRequest req) throws ConstructorException {
                 for (Map.Entry<String, File> entry : files.entrySet()) {
                     String fileName = entry.getKey();
                     File file = entry.getValue();
+                    String fileExtension = getValidatedFileExtension(file, fileName);
 
                     multipartBuilder.addFormDataPart(
                             fileName,
-                            fileName + ".csv",
+                            fileName + fileExtension,
                             RequestBody.create(MediaType.parse("application/octet-stream"), file));
                 }
             }
@@ -2365,9 +2413,12 @@ public String replaceCatalog(CatalogRequest req) throws ConstructorException {
     /**
      * Send a partial catalog to update specific items (delta)
      *
-     * @param req the catalog request
-     * @return a string of JSON
-     * @throws ConstructorException if the request is invalid.
+     * Supports CSV, JSON, and JSONL file formats. The file type is automatically
+     * detected from the file extension (.csv, .json, or .jsonl).
+     *
+     * @param req the catalog request containing files with .csv, .json, or .jsonl extensions
+     * @return a string of JSON containing task information
+     * @throws ConstructorException if the request is invalid or file extensions are not supported
      */
     public String updateCatalog(CatalogRequest req) throws ConstructorException {
         try {
@@ -2393,10 +2444,11 @@ public String updateCatalog(CatalogRequest req) throws ConstructorException {
                 for (Map.Entry<String, File> entry : files.entrySet()) {
                     String fileName = entry.getKey();
                     File file = entry.getValue();
+                    String fileExtension = getValidatedFileExtension(file, fileName);
 
                     multipartBuilder.addFormDataPart(
                             fileName,
-                            fileName + ".csv",
+                            fileName + fileExtension,
                             RequestBody.create(MediaType.parse("application/octet-stream"), file));
                 }
             }
@@ -2423,9 +2475,12 @@ public String updateCatalog(CatalogRequest req) throws ConstructorException {
     /**
      * Send a patch delta catalog to update specific items (delta)
      *
-     * @param req the catalog request
-     * @return a string of JSON
-     * @throws ConstructorException if the request is invalid.
+     * Supports CSV, JSON, and JSONL file formats. The file type is automatically
+     * detected from the file extension (.csv, .json, or .jsonl).
+     *
+     * @param req the catalog request containing files with .csv, .json, or .jsonl extensions
+     * @return a string of JSON containing task information
+     * @throws ConstructorException if the request is invalid or file extensions are not supported
      */
     public String patchCatalog(CatalogRequest req) throws ConstructorException {
         try {
@@ -2456,10 +2511,11 @@ public String patchCatalog(CatalogRequest req) throws ConstructorException {
                 for (Map.Entry<String, File> entry : files.entrySet()) {
                     String fileName = entry.getKey();
                     File file = entry.getValue();
+                    String fileExtension = getValidatedFileExtension(file, fileName);
 
                     multipartBuilder.addFormDataPart(
                             fileName,
-                            fileName + ".csv",
+                            fileName + fileExtension,
                             RequestBody.create(MediaType.parse("application/octet-stream"), file));
                 }
             }
diff --git a/constructorio-client/src/test/java/io/constructor/client/ConstructorIOCatalogTest.java b/constructorio-client/src/test/java/io/constructor/client/ConstructorIOCatalogTest.java
index 260b5513..a6e388cc 100644
--- a/constructorio-client/src/test/java/io/constructor/client/ConstructorIOCatalogTest.java
+++ b/constructorio-client/src/test/java/io/constructor/client/ConstructorIOCatalogTest.java
@@ -405,4 +405,213 @@ public void PatchCatalogWithItemsAndVariationsAndItemGroupsFilesShouldReturnTask
         assertTrue("task_id exists", jsonObj.has("task_id") == true);
         assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
     }
+
+    // JSONL Format Tests
+
+    @Test
+    public void ReplaceCatalogWithJsonlItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/jsonl/items.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.replaceCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void UpdateCatalogWithJsonlItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/jsonl/items.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.updateCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void PatchCatalogWithJsonlItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/jsonl/items.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.patchCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    // Invalid Extension Tests
+
+    @Test
+    public void ReplaceCatalogWithInvalidExtensionShouldError() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/invalid/items.txt"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+
+        thrown.expect(ConstructorException.class);
+        thrown.expectMessage("Invalid file type for 'items'");
+        thrown.expectMessage("must have .csv or .jsonl extension");
+        constructor.replaceCatalog(req);
+    }
+
+    @Test
+    public void UpdateCatalogWithNoExtensionShouldError() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/invalid/items"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+
+        thrown.expect(ConstructorException.class);
+        thrown.expectMessage("Invalid file for 'items'");
+        thrown.expectMessage("must have .csv or .jsonl extension");
+        constructor.updateCatalog(req);
+    }
+
+    @Test
+    public void PatchCatalogWithInvalidExtensionShouldError() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/invalid/items.txt"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+
+        thrown.expect(ConstructorException.class);
+        thrown.expectMessage("Invalid file type for 'items'");
+        thrown.expectMessage("must have .csv or .jsonl extension");
+        constructor.patchCatalog(req);
+    }
+
+    // Edge Case Tests
+
+    @Test
+    public void ReplaceCatalogWithMixedFileTypesShouldSucceed() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/csv/items.csv"));
+        files.put("variations", new File("src/test/resources/jsonl/variations.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.replaceCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void UpdateCatalogWithJsonlVariationsAndItemGroupsShouldSucceed() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("variations", new File("src/test/resources/jsonl/variations.jsonl"));
+        files.put("item_groups", new File("src/test/resources/jsonl/item_groups.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.updateCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void PatchCatalogWithAllJsonlFilesShouldSucceed() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/jsonl/items.jsonl"));
+        files.put("variations", new File("src/test/resources/jsonl/variations.jsonl"));
+        files.put("item_groups", new File("src/test/resources/jsonl/item_groups.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.patchCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    // JSON Format Tests
+
+    @Test
+    public void ReplaceCatalogWithJsonItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/json/items.json"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.replaceCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void UpdateCatalogWithJsonItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/json/items.json"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.updateCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void PatchCatalogWithJsonItemsFileShouldReturnTaskInfo() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/json/items.json"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.patchCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
+
+    @Test
+    public void ReplaceCatalogWithMixedCsvJsonJsonlShouldSucceed() throws Exception {
+        ConstructorIO constructor = new ConstructorIO(token, apiKey, true, null);
+        Map<String, File> files = new HashMap<String, File>();
+
+        files.put("items", new File("src/test/resources/csv/items.csv"));
+        files.put("variations", new File("src/test/resources/json/variations.json"));
+        files.put("item_groups", new File("src/test/resources/jsonl/item_groups.jsonl"));
+
+        CatalogRequest req = new CatalogRequest(files, "Products");
+        String response = constructor.replaceCatalog(req);
+        JSONObject jsonObj = new JSONObject(response);
+
+        assertTrue("task_id exists", jsonObj.has("task_id") == true);
+        assertTrue("task_status_path exists", jsonObj.has("task_status_path") == true);
+    }
 }
diff --git a/constructorio-client/src/test/resources/invalid/items b/constructorio-client/src/test/resources/invalid/items
new file mode 100644
index 00000000..a992b237
--- /dev/null
+++ b/constructorio-client/src/test/resources/invalid/items
@@ -0,0 +1 @@
+This file has no extension for testing validation.
diff --git a/constructorio-client/src/test/resources/invalid/items.txt b/constructorio-client/src/test/resources/invalid/items.txt
new file mode 100644
index 00000000..887c87ec
--- /dev/null
+++ b/constructorio-client/src/test/resources/invalid/items.txt
@@ -0,0 +1 @@
+This is a text file with invalid extension for catalog upload testing.
diff --git a/constructorio-client/src/test/resources/json/items.json b/constructorio-client/src/test/resources/json/items.json
new file mode 100644
index 00000000..69b7f47f
--- /dev/null
+++ b/constructorio-client/src/test/resources/json/items.json
@@ -0,0 +1,23 @@
+[
+  {
+    "id": "item1",
+    "value": "Product 1",
+    "data": {
+      "url": "https://example.com/product1"
+    }
+  },
+  {
+    "id": "item2",
+    "value": "Product 2",
+    "data": {
+      "url": "https://example.com/product2"
+    }
+  },
+  {
+    "id": "item3",
+    "value": "Product 3",
+    "data": {
+      "url": "https://example.com/product3"
+    }
+  }
+]
diff --git a/constructorio-client/src/test/resources/json/variations.json b/constructorio-client/src/test/resources/json/variations.json
new file mode 100644
index 00000000..45fb9621
--- /dev/null
+++ b/constructorio-client/src/test/resources/json/variations.json
@@ -0,0 +1,18 @@
+[
+  {
+    "id": "var1",
+    "item_id": "item1",
+    "value": "Product 1 Variation A",
+    "data": {
+      "color": "red"
+    }
+  },
+  {
+    "id": "var2",
+    "item_id": "item1",
+    "value": "Product 1 Variation B",
+    "data": {
+      "color": "blue"
+    }
+  }
+]
diff --git a/constructorio-client/src/test/resources/jsonl/item_groups.jsonl b/constructorio-client/src/test/resources/jsonl/item_groups.jsonl
new file mode 100644
index 00000000..2809502e
--- /dev/null
+++ b/constructorio-client/src/test/resources/jsonl/item_groups.jsonl
@@ -0,0 +1,2 @@
+{"id":"group1","value":"Group 1","data":{"parent_id":"root"}}
+{"id":"group2","value":"Group 2","data":{"parent_id":"root"}}
diff --git a/constructorio-client/src/test/resources/jsonl/items.jsonl b/constructorio-client/src/test/resources/jsonl/items.jsonl
new file mode 100644
index 00000000..b3c5f58b
--- /dev/null
+++ b/constructorio-client/src/test/resources/jsonl/items.jsonl
@@ -0,0 +1,3 @@
+{"id":"item1","value":"Product 1","data":{"url":"https://example.com/product1"}}
+{"id":"item2","value":"Product 2","data":{"url":"https://example.com/product2"}}
+{"id":"item3","value":"Product 3","data":{"url":"https://example.com/product3"}}
diff --git a/constructorio-client/src/test/resources/jsonl/variations.jsonl b/constructorio-client/src/test/resources/jsonl/variations.jsonl
new file mode 100644
index 00000000..5b17177f
--- /dev/null
+++ b/constructorio-client/src/test/resources/jsonl/variations.jsonl
@@ -0,0 +1,3 @@
+{"id":"var1","item_id":"item1","value":"Product 1 Variation A","data":{"color":"red"}}
+{"id":"var2","item_id":"item1","value":"Product 1 Variation B","data":{"color":"blue"}}
+{"id":"var3","item_id":"item2","value":"Product 2 Variation A","data":{"color":"green"}}