The dm_84 is a Wolfenstein Enemy Territory demo file extension. You can record your gameplay, which is saved as a movie file with dm_84 extension.
The files are stored in the “ET/etmain/demos/” folder. They can only be played ingame by using the “Replays” button in the game.
If you are playing in Jaymod mod for example and you have recorded your game, then it is saved in the “ET/etmain/jaymod/demos/” folder.
When you want to replay your recorded file, just start ET, click on MOD, click on Jaymod (this will restart your game in Jaymod mod), then click on Replays button to see your recorded movies. Recorded movies under Jaymod mod can only be seen, by starting your game in Jaymod mod. The same applies for the other MOD’s also.