I'm new in blender (I started in January 2018), this is the first time I try to get an animation and it still doesn't work "as expected". I used JPEGs as frames when I rendered first into animation, after I created a new file in which I added them in the video editing mode.There is (only) 100 frames and the preview looks good. When I render the animation in the video editing mode, I finally get a file that is neither a .MP4, or a .avi, or .mov BUT a sort of "Textedit document".
To fix it I already try to change the output, the codec, the quality & rapidity for the render. (even switched from blender render to cycles render too, to see if something different could happen)