There is a problem with the forum at the moment where attachments will be downloaded with random filenames instead of the proper names they should have.
For example, if you download this at the time of writing, you will get a file named 54b1acba53aad76163945a35f8a736b564016e15.zip instead of test.zip:
test.zip (41.4 KB)
If you try it and get a file named test.zip, you know the problem has been fixed.
The Discourse developers are aware of the problem and I'm guessing will fix it pretty soon:
For now, don't be surprised by the weird names, and remember to rename them after you download them, if you want to keep them and know what they are later.