Need an automated way to catch uppercase file extensions.

[megalon]

Issue:
Twice now an uppercase file extension on an image have caused builds to fail.
This is easy to miss visually, and the error does not occur on Windows, so it it's difficult to catch until trying to deploy.
On top of this, if you rename the file extension to lowercase in explorer, Windows doesn't actually update it.

See this failed build log
https://github.com/beat-saber-modding-group/wiki/commit/56f0e9c2de436f84699ec3e82d808e3a5472adee/checks?check_suite_id=387285740
And this PR
#33

Possible solutions:

• Add this to the build checks
• Add this to the linter