Requested feature overview description.
The ability to upload from view/full URLs on Furaffinity.

I.e. you go to https://e621.net/post/upload and put one of the following links in the "OR Upload from URL" field:
http://www.furaffinity.net/view/21788146/
http://www.furaffinity.net/full/21788146/

and e621 does two things:
a) Upload http://d.facdn.net/art/dargons/1479785259/1479785259.dargons_surge.jpg
b) Put these values in the post's Source field:
http://www.furaffinity.net/user/dargons/
http://www.furaffinity.net/view/21788146/
http://d.facdn.net/art/dargons/1479785259/1479785259.dargons_surge.jpg

Why would it be useful?
Many people upload images from direct FA links, and then don't bother to add the view/full URL to the Source field, so it ends up looking like this:
http://www.furaffinity.net/user/dargons/
http://d.facdn.net/art/dargons/1479785259/1479785259.dargons_surge.jpg

I know that you can use eSix Extend to find the image on FA if there's a direct FA link in the Source field. I suggested this feature and have used it many times :)
But if that could be done automatically, it would improve the quality of sourcing on e621.

This could be done with other furry image sites, too.
Like Inkbunny, where it would prevent people from uploading preview versions.
Then again, the IB admins wouldn't like this, because they think everybody who reposts art from IB is "stealing".

What part(s) of the site page(s) are affected?
https://e621.net/post/upload

+1, and all of my yes.

+100. If users upload 400 FA pics daily and take an extra 5 seconds to grab the image URL each time, that's 4 man-hours saved weekly.

\+1

I though that the source autofill worked from data on the url itself, which is why artist page is possible to fill in as it's part of filename in url. Also why on sites like tumblr it's impossible and deviantart it's possible to get all links.
FAs submission ID is global counting number, while direct image URL seems to contain artist nickname, timestamp and title, meaning that with submission page url you can't get the artist link neither.

So getting image url from the submission page would essentially require fetching whole page instead of just filling stuff from already available url string. Also would be inconsistant with other sites which still require the direct link to image instead of submission page.

+1 if it's possible to get working of course. Better would be to get submission page from direct image link.