To 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...