Return None from detect_mime for non-bytes input

[RensDimmendaal]

W this pr detect_mime returns None if the inputs are not bytes instead of raising an error.
It already returns None if no mimetype was found in the bytes input.

I noticed I was doing checks for isinstance(data, bytes) or putting it in a try/except block when using this function and I figured it might be better to handle this case in here instead.