Thumbnail previews of media files from URL, to be uploaded to a Wikibase (or Wikimedia Commons)

Scenario: User wants to use OpenRefine to upload new files to Wikimedia Commons, from the web. They select a set of URLs (or start a project from API, or similar).

We want to give users the option to toggle previews (thumbnails) of the media files, and the option to click the thumbnails to enlarge them. Such preview thumbnails are helpful during editing (e.g. to check if a certain thing is indeed depicted in a file, without having to preview the file via another application on the local drive itself).

Wireframes for this feature have been drawn by @lozanaross for another scenario (show thumbnails of files already on Wikimedia Commons), but I think the basic UX behavior is the same? OpenRefine/OpenRefine#5154 provides the technical basis for making this feature possible.

Most recent wireframes I found (v4, development version; shows files on Commons):

• The small thumbnails: https://xd.adobe.com/view/df767737-c4ca-4d25-977e-7ad8ca3a6840-8a25/screen/8c10c8d4-9ab6-4da2-adb6-9552f789593f/
• Hover effect: https://xd.adobe.com/view/df767737-c4ca-4d25-977e-7ad8ca3a6840-8a25/screen/e8aadeae-d6e6-414f-9852-26e85ce6964d/
• Enlarged thumbnail: https://xd.adobe.com/view/df767737-c4ca-4d25-977e-7ad8ca3a6840-8a25/screen/698c6f1c-d747-4adc-895b-142e21c2830c/

Review the linked wireframes and OpenRefine/OpenRefine#5154, which provides the stated technical basis. Determine the extension entry points for selecting URL-based media and displaying previews; done means users can toggle thumbnails and enlarge them while editing before upload to Wikimedia Commons.