Deck files format

When you load a deck, what file formats are "Deck files" that's in the file type box looking for?
I am drafting on a third party site with the available file types of .txt .Mwdeck .cod
No files show up under those formats when I try to look for them. How ever when I type in the file name (file.mwdeck) it shows up and loads...
I am drafting on a third party site with the available file types of .txt .Mwdeck .cod
No files show up under those formats when I try to look for them. How ever when I type in the file name (file.mwdeck) it shows up and loads...