Discard non videos from the sources for the video preview

The AI project uses the "conversions" endpoint to generate audio files and store them to the vid:transcodedVideos metadata.

And because of that, the audio file is sometimes used for the preview. Therefore no video is displayed.

Even if this approach is debatable, the element "nuxeo-document-preview" could be improved to filter out the non videos MIME type from the sources of the preview around these lines https://github.com/nuxeo/nuxeo-ui-elements/blob/f03857c5cbe5d49bfca1c06b6a3bf118f1471679/nuxeo-document-preview.js#L254-L256