Fix for a race condition in process_file(). 

If a hash was not in Redis when process_file() was executed, processing would fail without deleting the hash from the database.

It may be better to use a loop on this (while f == None) but I figure infinite loops should be the author's domain. As it is, if the hash is not in Redis after 2 seconds, the failure mode will be as before: the processing will fail and *then* the hash will be added to the database, without any processing. This will result in the "Unable to detect file type" error page whenever that file is uploaded.