forgotten-attachment: fix empty regex expression

forgotten-attachment: fix empty regex expression

The original regex was of the form "abc(|def)" to check for both "abc"
and "abcdef".  Unfortunately, the regex libraries on BSDs/MacOS don't
like this use of an empty sub-expression.

Expanding the regex to: "(abc|abcdef)" fixes the problem.

Fixes: neomutt/homebrew-neomutt#15