files: Attempt to standardize thread entry attaching

This commit attempts to remove some of the confusing and redundant code to
attach files to thread entries and replace it with a single code base. It
also attempts to remove and error where a single attachment might be
attached to a new thread entry multiple times.

Lastly, it removes the insert followed by an update for emails with inline
images. This should improve performance processing emails as only one trip
to the database is now necessary for thread entries with inline images.