File Types Blocked in Gmail
support.google.comTo the file system, all files are instances of the same type. Google's notion of "file type" is likely based on the extension, which is only a naming convention. If they're peeking inside at the contents to discern a signature, that's a security problem.
I had a password-protected .7z file blocked for security reasons, so I changed the file extension to .jpg to see if it would be accepted, and it was not.
Edit: I think all file extension issues can be bypassed by converting a file to an ASCII-armored format with GPG or age or similar...