Improve error handling of unplayable videos

⚠️ This issue respects the following points: ⚠️

• This is a bug, not a question or a configuration/webserver/proxy issue.
• This issue is not already reported on Github OR Nextcloud Community Forum (I've searched it).
• Nextcloud Server is up to date. See Maintenance and Release Schedule for supported versions.
• I agree to follow Nextcloud's Code of Conduct.

Bug description

I can't play videos from within Nextcloud (tested on c.nc.com)

Tested with Firefox 141.0 on Linux and Chromium 138.0.7204.183

Steps to reproduce

1. Navigate to a video (in my case a mkv-file)
2. open it

I will see a spinner but the video never starts:

Interestingly the talk desktop client seems to have a different player build in. The Talk desktop client plays at least the audio, but the video stays black.

Expected behavior

Nextcloud plays the video in the browser

Nextcloud Server version

31

Operating system

None

PHP engine version

None

Web server

None

Database engine version

None

Is this bug present after an update or on a fresh install?

None

Are you using the Nextcloud Server Encryption module?

None

What user-backends are you using?

• Default user-backend (database)
• LDAP/ Active Directory
• SSO - SAML
• Other

Configuration report

List of activated Apps

Nextcloud Signing status

Nextcloud Logs

Additional info

No response

No source file or test is named. First reproduce the spinner with the reported MKV video in Firefox and Chromium, then trace the viewer's browser video-playback entry point and inspect available logs. Done means the video starts in the browser instead of remaining on the loading spinner.