Hi Babydoll.
AVI Files are Video Files. MP3 Files are Audio Files. What exactly are you trying to do??
Here is a list of "Common File Extensions" that may help you understand what you may see on the Network.

Common File Extensions
Hope this Helps.

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I Don't Suffer From Insanity, I Enjoy Every Minute of it

hi,when i search for a song for example a list comes up,and under TYPE shows mp3. but before they used to come us as something different.the mp3 doesnt download for me.only half or part of the song.i wanted to know is there a way to change this.

And that's just the very very most common. I am surprised how many extensions that list doesn't include, such as apple's m4a audio format or the almost equivalent mp4 (so I wonder how old that list is). But this list will help you with the most basic file extensions.

(this is a good way to find out what a particular file extension might do FILExt - The File Extension Source )