Release 10: Upload of thumbnail files does not work

[dagraf]

Describe the bug
When I upload a video and want to upload a thumbnail file the upload runs into an error (see screenshot)

To Reproduce
Steps to reproduce the behavior:

1. Go to a series and click on 'Upload Event'
2. Select a video and a thumbnail file (VTT)
3. Click on 'Save'
4. See error: First, the spinning wheel appears while the video gets uploaded to ILIAS. When this upload is finished, the error message appers

Screenshots
This error is shown:

Environment (please complete the following information):

• ILIAS version: v10.6 2026-03-10
• Plugin version: 10.1.0
• Browser: Firefox
• Opencast version: 20