t_v_m: dont escape filename when saving attachments

Extra Shellwords.escaping was introduced in 3106f6d where in reality only
functionality for saving with default filenames should be added.

The Shellwords escaping is removed. The filename is not used in any command,
and any program or command out-of-sup needs to account for correct filename.
The filename may potentially contain shell commands, the file will be saved
with the filename indicated by the SENDER of the email.

Fixes #277.