oops: Fix finding key of existing file

If a file is attached via email and sent into the system, and a file is on
record with the same signature (hash) and size, the system will not save the
file. Instead, the key of the existing file would be found and used instead.

This patch fixes a bug in AttachmentFile::save. The key was generated for
the new file; however, if it was determined to be a duplicate, the key of
the existing file was not returned. Therefore generated key, which wasn't
saved to the database, was returned. Therefore, the wrong key was placed in
the body of the message with cid:<key> for inline images, although that key
would not exist in the database.

This patch correctly returns the existing key from the ::save() method
for de-duplicated files.