How to extend Fileupload dialog with required description

Hi!

I have a case where my customer needs to have required description on each image uploaded to episerver. We have extended ImageData and added a description field on this where the editor add the description if they remember. But how can we set the Description field required so that editors needs to add the description for each file. I think to extend the fileupload dialog will be the cleanes solution, but have not figured out how.