Requested feature overview description

When initially uploading a post, or when editing an existing post, disallow source URLs that have the file: scheme in some way.

This might consist of quietly dropping the source, or dropping the source and adding a tag like invalid_source, or even refusing to accept the upload or edit until that URL is removed. It should probably follow whatever existing action (if any) is currently done for an "invalid" source.

Why would it be useful?

.file: URLs can only ever point to something local to the user's computer; they don't correspond to anything that anyone else can view or download from a server somewhere. Sometimes they may contain the artist's name, or a title for the image, but that's not guaranteed.

A fully-qualified file: URL may also "leak" the uploader's Windows account name, which the uploader may wish to avoid.

Disallowing the URL may alert an uploader or editor that they should find a Internet-visible link as a source, rather than a link that is local to their computer.

There are currently at least 180 posts with a file: URL, per a search. I checked an e621 database dump from July 2022 and there were about 211 file: URLs as sources, but some of those may have been edited since then. Of those 211, 4 of them contain a Windows user account name.

What part(s) of the site page(s) are affected?

The upload page and the "edit" function on each existing post.