ugh I hate that copy-pasting an image region out of Paint.NET into mastodon will upload (and mastodon will then serve) a PNG.
My camera takes big pictures! I don't want to upload 11mb PNGs that download slower.
The worst part is that I had a fix for this, I wrote myself a tool to JPEGify the clipboard... but it was on my old desktop, and I never moved it to my laptop because it was kinda jank and I wasn't sure how to unjank it.