Preserve query string params when downloading imported attachments
ImportLog needs to be on a separate db connection too

Now that logs are not in the import model directly (see e287304d0a8a3632dcc4355c8a97db06b210f0c5) they need to be explicitly set to use a new DB connection in Import.use_separate_connection otherwise a failed import will rollback the transaction and wipe the entries in the log.
Let the attachment cache identify more Word docs
Use Content-Type header when downloading attachments

If the downloaded file has no extension (either in the url or via Content-Disposition) we look for the first extension from MIME::Types for the Content-Type and use that.  Only if that is missing do we fall back to our old `file ...` implementation which only has a few rules.
Always use the fake rummageable implementation in test