How to make a 3gpp file usable for the movie clip editor ? I have a movie file in the 3gpp format (shot with android smart phone, using Linux).
I switched into the movie clip editor and clicked on the open-button to import it as footage. Yet this file is not displayed in blenders file browser. 
How do I continue from here?

Should I convert this file into another one, and if yes with what program into what format ? 

Should I use an addon for blender if possible ?
This question comes up pretty high in searches so I thought I'd add what worked for me - converting it to mp4 with ffmpeg:

ffmpeg -i originalvid.3gp -q:v 0 -q:a 0 -strict -2 targetvid.mp4

This keeps the video (q:v 0) and audio (q:a 0) at the maximum possible quality.