I recently started having problems with downloads in Opera on my Mac. I see others have had this issue, so I wanted to post the solution that worked for me. For me it's a Mac issue!
I recently upgraded my Mac to Mojave. I had to go into System Preferences, Security and Privacy, Privacy tab, in the left column scroll down to "Full Disk Access", unlock (bottom left), click the small plus under the right side, and add Opera.app to Full Disk Access.
Now things download and get the right name. Hope this helps others!