Make the file extension check for .AppImage case-insensitive.

The problem is that we often have .AppImage instead of the more Unixy .appimage, but the later occurs too.

So, We'll tolerate whatever CrAzY Casing you can come up with.

Resolves lutris#5396

lutris/util/extract.py

@@ -89,7 +89,7 @@ def guess_extractor(path):
         extractor = "exe"
     elif path.endswith(".deb"):
         extractor = "deb"
-    elif path.endswith(".AppImage"):
+    elif path.casefold().endswith(".appimage"):
         extractor = "AppImage"
     else:
         extractor = None