It's being treated as a video fine, but isn't playable in the browser. This typically means the encoding in the video is wrong and not in a format that's playable in the browser. CB Gallery does not perform any processing on video or audio files so it is not capable of correcting encoding issues. 99% of our users don't have the server capabilities for processing video or audio, but we do plan to implement support for ffmpeg in the future to allow for this.
See the below regarding browser safe formats, encodings, and limitations.
developer.mozilla.org/en-US/docs/Web/Media/Formats/Video_codecs
Note I highly recommend offloading the videos to youtube.