* Save the filename for files uploaded from disk. This could be used in
the future to extract source data if the filename is from a known site.
* Save both the image URL and the page URL for files uploaded from
source. This is needed for multi-file uploads. The image URL is the
URL of the file actually downloaded from the source. This can be
different from the URL given by the user, if the user tried to upload
a sample URL and we automatically changed it to the original URL. The
page URL is the URL of the page containing the image. We don't always
know this, for example if someone uploads a Twitter image without the
bookmarklet, then we can't find the page URL.
* Add a fix script to backfill URLs for existing uploads. For file
uploads, the filename will be set to "unknown.jpg". For source
uploads, we fetch the source data again to get the image and page
URLs. This may fail for uploads that have been deleted from the
source since uploading.
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