Fix attachment filename handling

Attachment#write_to_disk created its temporary file with a shell-escaped name.
Passing the path to this file to write_to_disk's block parameter and returning
it created a double-escape problem, which rendered attachments with e.g. spaces
in the filename unusable.

Fixed by removing the use of the original filename from the tempfile name. Also
removed the warnings about shell escaping the temporary file name.