I second that! I do sketches and drawings on MyPaint... and I name my finished drawings just with date. So if the scrap file is like that I don't need to manually save all things, just save a scrap and you can go back whenever you want and then it can also be a finished drawing as well.

the numbers and letter are simply a single serial number (think of the letter as being a number 0-25), you can see this in the implementation, line 1293 of gui/filehandling.py. Personally I like to use the date, so I patched my mypaint to preprocess the prefix with time.strftime before passing it to save_autoincrement_file. But it's not safe to use only the date, consider what happens if you want to save two scraps on same day - the second one overwrites the first. Date and time has the same problem, too, it's just much less likely to want to save in the same minute than the same day.

The modification I mention avoids this and you end up with the date followed by the 'numbers then a letter' which you were already getting.
The other issue is making sure it sorts easily in the file browser. If you want that then the date must be in the order [Year, Month, Day]. The 'numbers and a letter' already sort correctly with standard alphanumeric file browser name sort.

Personally I doubt there is a solution that everyone will be happy with. strftime is okay with me because I already know the format codes. Maybe people will be ok with having the date included by default. I suspect they will want the date order they are familiar with. If that's the case then we would need to use something which is at least similar to strftime to allow the customization of format.
It can't just be taken from the locale info for various reasons (internal or personal standards may not match country standards).

This is what I came up with:

The automatic setting will output something like scrap_01-12-2022_001_a.ora and the default is the same as before but can interpret time.strftime directives now.