I don't think we should reuse schema:about to link file description page and MediaInfo entity, as it is already used by sitelinks. In theory file description page may have its item (though it's not allowed in Wikidata), which will make schema:about ambigous.
Topic on Extension talk:WikibaseMediaInfo/RDF mapping
That's indeed a good point. I believe there somehow the same relation between file description page and media info entity as between e.g. wikipedia articles and items. So, I don't think it is bad to reuse the same relation but using a more specialized one might make querying easier indeed. Do you have an idea of an other relation from schema.org or elsewhere to use instead?