Before we were doing str(time.time()). time() returns a float, and str()
always rounds the part after the decimal. This means that two emails
sent close together could be given the same filename, causing the
second one to overwrite the first.

If we use "%f" instead, then you get full accuracy.
Merge pull request #1 from tindie/fix/unique_filenames

Fix problem of quickly sent emails being overwritten