Fix non-html stuff hitting Checkpoint

This reverts commit c666415b80.

checkpoint.js

@@ -737,8 +737,9 @@ function CheckpointMiddleware() {
           }
         }
 
-        // Skip checkpoint for requests that don't accept HTML
+        // Only checkpoint requests explicitly accepting 'text/html'
-        if (!req.accepts('html')) {
+        const acceptHeader = request.headers.get('accept') || '';
+        if (!acceptHeader.toLowerCase().includes('text/html')) {
           return next();
         }