mailsave_to_filefield module needs testing

here is a quick and dirty version of mailsave_to_filefield borrowed from the mailsave_to_imagefield module.

it works for me but probably needs testing.

it also adds an admin setting to chose a mailbox to hook into, thus not breaking other mailboxes for content types that dont have filefields.

this setting should be expanding to allow multiple mailboxes, but I don't need this right now.

thoughts?