Currently we have some magic bytes at the beginning of the thumbnail (handled by ReplayStudio), we should probably get rid of them while still being able to read the old format.

Badlion emits replay files without the magic bytes (which tbh is the reasonable way) which caused #201 for us. If we just change to not include the bytes as well, then we'll also crash RM versions prior to the one fixing aforementioned issue if people need to downgrade for some reason.
So, to make the upgrade as smooth as possible, we should:

• Write new thumb files under a different name and be able to read thoes
• Retain the ability to read old thumb files for backwards compatibility
• [Bonus] Be able to read old thumb files without the magic bytes (for better compat with badlion replays until, if, they switch to the new thumb file name)