Most media file formats support more than one filename extension (e.g. ogg/ogv, mp4/m4a, mkv/webm, ts/m2t/mts). I do not maintain a set of valid extensions for each format you see in that format list. Extension may also be based on whether audio or video were turned off making it more complicated. The automatic filename implementation is intentionally simple, and I will likely not be changing it. If you enter a period you should supply a file name extension. You can also rename it after the fact if you notice it. Finally, file name extensions are not strictly necessary but usually very helpful.