Most Opencast workflows will take snapshots of the media package where the media package elements to be included in the snapshot usually are identified by tags. Most commonly, the tag 'archive' is used for that.
If the user uploads a thumbnail using the Video Editor, the uploaded image will be included as attachment in the media package with the flavor 'thumbnail/source' (default configuration).
Since that attachment does not have the tag 'archive' assigned, running a workflow that performs a snapshot operation based on the tag 'archive' will cause the uploaded image not to be included anymore in the newly created snapshot.
Steps to reproduce:
1. Upload thumbnail image in the Video Editor
2. Look at the media package (Events->Event Details->Assets->Attachments)
3. Run the workflow "Update previews" for the event you have uploaded the thumbnail
4. Again, Look at the media package (Events->Event Details->Assets->Attachments)
The attachment 'thumbnail/source' is not in the media package anymore in step 4
The attachment 'thumbnail/source' should still be included in the media package.