Hello there, would it possible to extract description data also from another two sources: 1, description of the image in the gallery section of a page, and 2, plain text in the text body of an image (content of the page File:Image.jpg) ?
Topic on Extension talk:CommonsMetadata/Flow
1. is hard, and we don't want to put much effort into parsing data from wikitext, the plan is to support structured data via Wikibase (Wikidata) instead. As for 2., it would not be very useful without locating out the useful part of the description (there will be license templates, section headers and whatever else intermingled with it) which is again more effort than worth. (If you do need the contents of the file page though, you can get the via the revisions API.)