We're having the same problem with uploading .webm files both manually and via the API. Unfortunately our app can't easily run ffmpeg so this is a major problem for us. Daniel - is this a known bug that your developers have documented and are working on?
Can you perhaps send me the video file? I get an access denied error when I visited the link you shared above. You can submit it through the ‘Contact Support’ link in your dashboard if you don’t want to expose your content to the public forum.
Alternatively, can you re-encode your content in mp4 format with Handbrake and then try to re-upload the content?
Here is a helpful document to assist you if you have not yet seen it: https://support.jwplayer.com/customer/portal/articles/1433634-preparing-videos-for-upload
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