mailcap_sanitize should strip '-' if the first character of filenames

I noticed that mailcap_sanitize allows '-', if it's the first character of a filename it could be accidentally interpreted as a parameter.

I think it would be safer to disallow it as the first character, anywhere else is fine because whitespace is already sanitized.

(I almost made this mistake in a mailcap helper script, but luckily decided to check the whitelist)