LibWeb: Reverse check for whether a FilesList index is supported

This fixes for..of iteration of a FilesList object.

Tests/LibWeb/Text/input/input-file.html

@@ -13,7 +13,13 @@
                     const file = input.files.item(i);
                     const text = await file.text();
 
-                    println(`${file.name}: ${text}`);
+                    println(`${file.name} (index iteration): ${text}`);
+                }
+
+                for (let file of input.files) {
+                    const text = await file.text();
+
+                    println(`${file.name} (for..of iteration): ${text}`);
                 }
 
                 resolve();