What this converter actually supports
Convertr uses the deployed FFmpeg toolchain for published audio and video routes. A container such as MP4, MOV, MKV, AVI, or WebM can hold different codecs, so the file extension alone does not guarantee that a particular player will accept the contents.
Only media-like SWF content can become a useful video; MP4 is the practical playback target.
The public upload limit is 1 GB. Convertr does not reduce dimensions, duration, resolution, or output quality merely to manage worker load.