Re: Image mode

[Richard Stallman]

    Files for which the user is likely surprised that they are image
    files.  It seems like inappropriate file extensions (either those
    suggesting a different major mode, or those suggesting nothing at all)
    would be a good indicator.

I didn't believe this before, and I don't believe it now.
There is no rational reason to assume that an extension of .jpg
means the file can't do harm.  Most users do NOT feel suspicious
of files on account of a name ending in .jpg.

If indeed it is safe to open images if and only if their extensions
say they are images, the reason would have to be something more subtle
than what people have said here so far.