Feature proposal: set extension (.gif, .mp4) in file path selection popup to auto add extension if needed

[hsandt]

Currently, I must type the extnsion (.gif, .mp4) manually each time or I will export a file with a pure basename, and need to add the extension later.

Most apps open a file path prompt that has a set of extensions predefined, and will auto-fill the extension if not manually written in the filename. Since there is a dedicated button for GIF and MP4, we could just set the extension to the corresponding one.