Hi Bense,
I tried with a larger video than yours. It works.
Are you trying to preview the webm file on Safari?
Safari doesn't support webm files, so you can add mp4 files to the property panel:
Sources assets/xxx.mp4, xxx.webm
Also, webm is supported by Chrome and Firefox on Mac. Click on the drop-down icon next to the Preview button
and select other browsers to preview.
Thanks,
Rika