When guessing an attachment type, don't allow text/plain if there is a null character...

When guessing an attachment type, don't allow text/plain if there is a null character. (see #2933)

Type text/plain should not contain any null characters.  Slightly
improve the type guesser by forcing an attachment with any null
characters to be application/octet-stream.

Note the type guesser could use much more improvement, but this is an
easy and obvious fix.