undefined - Follow Up Deployment

index.html

@@ -416,7 +416,15 @@ async function submitAssignment(){
 
 /* ---------- VERIFY / DECRYPT (Teacher) ---------- */
 async function verifyAssignment(){
-    const file=assignmentFile.files[0];if(!file){alert('Please select a file');return;}
+    const file=assignmentFile.files[0];
+    if(!file){alert('Please select a file');return;}
+
+    // locks obvious mistakes to enforce the extension
+    if (!file.name.toLowerCase().endsWith('.task')) {
+      showVerificationResult(false,
+        'Wrong file type – please upload a ".task" assignment file.');
+      return;
+  }
     const reader=new FileReader();
     reader.onload=async e=>{
         try{