Requested feature overview description.

1. User uploads from a URL
2. User clicks "Upload"
3. e621 checks to see if the direct image URL is in the source
4. If not, and there are 4 or less source lines, add the direct image URL to the last line

Why would it be useful?

In some cases, direct Tumblr links are accessible even after the post or Tumblr has been deleted.

Example: post #765809. The tumblr post link is broken because the blog was censored by Tumblr (not the artist), but the media.tumblr.com direct link still works fine.

This may apply to other websites.

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

Post upload

DelurC said:
You can do the same for pixiv then too. (replacing part of string to get original image instead of preview)

For example
https://i.pximg.net/c/128x128/img-master/img/2016/12/29/10/10/25/60625109_p0_square1200.jpg
https://i.pximg.net/img-master/img/2016/12/29/10/10/25/60625109_p0_master1200.jpg

get changed to
https://i.pximg.net/img-original/img/2016/12/29/10/10/25/60625109_p0.jpg

"/c/128x128" deleted if it exists
"/img-master/" replaced with "/img-original/"
"_square1200" and "_master1200" deleted (there are multiple ways to deal with those)

After that is done, check if .png version exists, if it doesn't then use .jpg

Isn't this already a thing? Some sites like DeviantArt also autofill the artists page in the source field as well.

That's why sites like FA are bad because direct URL can autofill direct and artist page, but not submission.

upload by URL puts the direct url last if possible from my experience (i mainly use that)

Blind_Guardian said:
upload by URL puts the direct url last if possible from my experience (i mainly use that)

If that is true I would have missed it since I put the URL in the source myself.