While testing the chunked upload with an artificially inflated 1x1px file, I noticed that the thumbnail are broken. After checking for failed GET requests in the net tab and opening them (the URL is /wiki/Special:UploadStash/file/<stash_filekey>), I get this error:
Internal Server Error
Cannot serve a file larger than 1048576 bytes.
which suggests that UW tries to fetch this file in its full size to show a tiny thumbnail. That doesn't really make sense for large files. (Modern browsers probably wouldn't need to get the thumbnail from the client side in the first place.)