Requested feature overview description.
If a user flags their own post with the reason given being "I'm the uploader and I uploaded the file by mistake," and the post is deleted as a result, the deleted post will not be taken into account when calculating the user's upload limit.
To avoid abuse of the feature, the deleted post is only ignored if "I'm the uploader and I uploaded the file by mistake" is the reason a janitor or moderator cites for deletion (in cases where other users have flagged the post as well).

Why would it be useful?
This would reward users who are keen in spotting their own errors and notice that their post shouldn't have been uploaded, like if they see that the image is a duplicate after it's been uploaded. In addition, it would not punish users who make an honest mistake and accidentally upload something.

What part(s) of the site page(s) are affected?
The upload limit calculator

-1. It's worth keeping the penalty there to encourage you to make sure you've got the right image before you hit submit.

+1

@Furrin Gok
true thru nether should one be punished for correcting their own mistake promptly, probably have the exception kick in for 24 hours the same way uploaders are able to flag for any reason( write-in custom reasons) for the first 24 hours.

+1, but if they flag another post of theirs within 5 minutes of the original flag, then they are penalized.

I'm not sure why, but I was under the impression that this was already a thing.

leomole said:
I'm not sure why, but I was under the impression that this was already a thing.

Only if you flag it with the 48 hours reason. This is normally a thing. The only evidence against this that I've seen is two months ago, on their delete count, so it might be a fluke.

Siral_Exan said:
Only if you flag it with the 48 hours reason. This is normally a thing. The only evidence against this that I've seen is two months ago, on their delete count, so it might be a fluke.

I remember being told that it never was a thing?

It was never a thing, the deletion calculation doesn't care why something was deleted, it always counts against the uploader.