Send extra attribute from CKFinder to CKEditor 5

Question

My goal is simple: I want to add extra attributes to an img tag in CKEditor; e.g.: <img src="path" data-attr="myattr">.

I built my Python connector; I added my upcast/downcast to CKE, but I still don't fully understand how to send the data-attr attribute from CKFinder to CKEditor.

Pipeline should be:

• Click CKFinder
• Select an image — this should contain an extra attribute: e.g.: data-attr
• Add the image to CKEditor — the tag should contain the data-attr attribute

I can't figure how to do the last two steps. Any suggestions?