Incorrect assumption that all media_fields are fixed fields

[pvliesdonk]

Description

Fixes #5580

(...)

To Do

• [x] ~Documentation. (If you've added a new command-line flag, for example, find the appropriate page under docs/ to describe it.)~
• [ ] Changelog. (Add an entry to docs/changelog.rst to the bottom of one of the lists near the top of the document.)
• [ ] Tests. (Very much encouraged but not strictly required.)

[github-actions[bot]]

Thank you for the PR! The changelog has not been updated, so here is a friendly reminder to check if you need to add an entry.

[snejus]

How is this going to prevent the OperationalError that you've seen, where the custom field is not found?

It would be good to include a note in the changelog as well