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It should be possible in FirstSpirit Content Creator to edit the description field of newly added media like it is already possible in SiteArchitect.
this feature request is now almost 5 years old and as far as I can see this basic functionality is still not available in ContentCreator.
The field in the background to store the data is available because it is used in the SiteArchitect. so, it should "just" be an entry field in the frontend of the ContentCreator.
Since you currently enhance the ContentCreator with a lot of features that where only available in the SiteArchitect I think this should be included in one of your next releases!