Could you try upgrading your instance to latest and see if the problem persists? (since this won’t automatically fix the filenames of existing uploads, my bad, misread a followup on the linked topic, see @michaeld’s comment below)
I do a migration from one forum engine to Discourse and this problem has been following me since beta9 approximately. Seems that only new deployed image is free of the problem of the naming of downloaded files.
Actually, I don’t know where I did a mistake, and how to report this problem right way. I just deployed the latest Discourse image, then migrated attachments from an old forum to the new one via API, then both migrated and new uploaded files are downloaded with <hash> in the filename.
I know, my case is specific, therefore my post is just a little confirmation that the problem still persists in some cases.