Embedded media not processed by the multimedia filter in Workshop submissions

Steps to reproduce:

1. Make sure the multimedia filter is enabled at the site and the course.
2. Create a workshop and switch it to the submission phase immediately (no need to define the assessment form to test this).
3. Start preparing a submission.
4. Embed a media into the submission content editor (like some MP3 file or whatever the multimedia filter replaces with the in-built player).
5. Save the submission.

Expected result: Embedded file is replaced by the appropriate player by the multimedia filter.

Actual result: Only the link to download the file is displayed as if the multimedia filter was disabled.