Specify behavior when only slashes or backslashes are passed as filename parameter

[Phillip9587]

I'm opening this PR to discuss how we should handle filenames consisting only of slashes or backslashes. When a filename contains only slashes, the function returns an invalid header in my opinion:

attachment; filename*=UTF-8''

Is this the desired behavior, or should we handle it differently? Should we add explicit validation to reject invalid filenames?

[blakeembrey]

Is this the desired behavior, or should we handle it differently?

I think it's fine and an expected edge-case.

Should we add explicit validation to reject invalid filenames?

This is the more interesting question. Additionally, should we even basename file name at all? I'm not confident at this time to validate file names, since MacOS supports / and \ in their filenames (which must be breaking today if someone tried to specify it as filename.