This reduces chances of errors where consumers of strings mutate inputs
and reduces memory usage of the app.
Test suite passes now, but there may be some stuff left, so we will run
a few sites on a branch prior to merging
* Try multiple filenames and do lots of guessing when searching for attachments
* Unescape HTML in filenames and replace invalid characters in filenames
* Existing permalinks prevented resuming of import
* Prevent duplicate attachments in same post
