Properly handle DnD targets
The current drag-and-drop system from the preview image allows only the raw image buffer to be sent, instead of discriminating between the type of target, which often requires a well-formed file (as already generated by the "Copy to clipboard" feature) instead, especially in e-mail, web pages and chat apps (Telegram, Fractal...). This could be solved either by using the same source of the "copy to clipboard feature" (with the downside of not allowing the raw buffer to be processed anymore) or by using such pre-generated .png file only as fallback where the target does not allow raw image buffers to be dropped.
I have not looked into the code yet, so feel free to correct me if anything I said is wrong.
Cheers.