I have copied the image plugin to customize it upload video, which is working fine.
but when I select upload - it defaults to selection of image files, I tried searching everywhere, but couldn't find,how I can change this to type mp4 or of type video files.
TinyMCE has the Media plugin which allows you to natively upload video and audio files. It should work without any customizations to the Image plugin, thus - safer. However, to be able to upload something other than images, you will need to add a file_picker_callback that will handle uploads.