7z is a file compression format like a "zip" file.  Its name comes from the freeware program "7zip" that can be used to convert 7z files back to their original form, in this case an avi file.  The reason: Links at mediafire were being deleted quickly, for various reasons.  To avoid that, the files here are now placed in a 7z wrapper to disguise their content before being sent to mediafire.  

What you need to do to return the downloaded file(s) back to its original form:

1. Get and install 7zip from downloads.cnet.com  If you already have 7zip, it has to be version 9.2 or later.
2. Highlight the file downloaded from mediafire
3. Right click on the highlight and choose 7zip "Extract Here"
4. Enjoy

Note: If the file is bigger than 200MB, it is split into two or more parts as MediaFire requires.  In that case:

1. Highlight all the files to be joined
2. Right click on the highlight, choose 7zip "Extract Here"
Enjoy

Every computer should have a compression utility, and 7zip is a good one to have because it works with all the major compression formats and is completely free: opensource, no ads.

(*downloads.cnet.com is long established archive hosted by a consumer awareness group that makes efforts to be sure the files you download are free of viruses or spyware and are what they claim to be.)

If all of this is difficult for you, as of now only mediafire downloads have this procedure.  The other download sources just produce the avi file you are looking for.  Of the remaining download options, try PeejeShare first if it is available to you.  It is faster than mediafire where I live.  Next in line for speed is Sendspace.  And neither of these require you to solve a "captcha" or similar nonsense.  After that I would try JumboFiles, but it is not as fast or consistent as these first two.  

Admin is just trying to make things work.  It's either this or no mediafire at all.  Address your complaints to those responsible for killing MU, wpupload, Fileserve, Filesonic...