Code review comment for ~stgraber/simplestreams:master

As we have two different images files both with the ".img" extension, the previous logic was mistakenly selecting the wrong file, causing duplicate entries which were causing LXD to ignore the image entirely.

This fix corrects that by selecting the full file extension instead.

As we have bad image entries out there right now, we'll want the fix to land quickly AND a re-generation of the index to happen so that the bad entries go away and LXD stops ignoring those images.