don't rely on cached metadata from database for attachment info

This commit removes use of LocalFolder.MessagePartType as an indication
of part types in AttachmentInfoExtractor. The enum was intended as a
caching mechanism to skip loading unimportant parts from the database,
but only the reading side was implemented. As the expected performance
gain is negligible, I disabled the reading side for now (computing the
data instead) and more explicitly disabled the writing side as well.