The media/file descriptions contained in the dump are the wikitext of the revisions of pages with the File: prefix, plus the metadata about those pages and revisions (user that made the edit, timestamp of edit, edit comment, and so on).
Width and hieght of the image, the media type, the sha1 of the image and a few other details can be obtained by looking at the image.sql.gz file available for download for the dumps for each wiki. Have a look at
https://www.mediawiki.org/wiki/Manual:Image_table for more info.
Hope that helps!
Ariel Glenn