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

Scenario: User wants to use OpenRefine to upload new files to Wikimedia Commons, from harddrive. They select a set of files from their harddrive.

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. In the backend

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/

Start by reviewing the linked wireframes and OpenRefine#5154, which the issue identifies as the technical basis. Trace how local files selected for Wikimedia Commons upload are represented in the extension, then determine the UI and backend entry points involved. Done means users can toggle thumbnails and enlarge them while editing the selected files.