.dmg file renamed
Mac 12.0.1 User here. While attempting to upgrade from 94.01 to 94.02(and now seriously wondering why) I clicked on the downloaded .dmg file to open it and I was informed that there was no app capable of doing so. Then just to be mean, the file name was changed from awholebunchofletters&#s.dmg to awholebunchofletters&#s.dmg.part
...and not by me.
Firefox adds .part to file during the download stage and should remove the .part once the download has finished. If you keep the .part appended to the filename then this means that Firefox wasn't able to finish the download properly. This can be because security software kept a lock on the file. If no errors were reported during the download then renaming the file and remove the trailing .part work make the file work properly.