For an email file attachment it quite clearly shows it uploading.

The issue with the Instagram model is that it looks like you are just doing things locally on your phone, and haven't uploaded yet. If you have second thoughts and decide not to post the pic, Instagram could keep the photo.

That said, I don't really see why they would do this. What would it gain them to keep millions of discarded photos?

Iirc, they were saved as “drafts”, meaning you could come back to it later or on another device. Youtube was like that before instagram, it’s a popular feature on Snapchat/Tiktok.

My point is that it is feature clearly people appreciated, not a dark UX pattern.