LibWeb: Do not blindly create File objects when adding FormData entries

We were unconditionally creating new File objects for all Blob-type
values passed to `FormData.append`. We should only do so if the value is
not already a File object (or if the `filename` attribute is present).

We must also carry MIME type information forward from the underlying
Blob object.

Tests/LibWeb/Text/input/HTML/FormData-append-blob-with-type.html

@@ -0,0 +1,13 @@
+<script src="../include.js"></script>
+<script>
+    test(() => {
+        const blob = new Blob(["Hi Blob!"], { type: "text/plain" });
+
+        const formData = new FormData();
+        formData.append("test", blob, "test.txt");
+
+        const file = formData.get("test");
+        println(`Name: ${file.name}`);
+        println(`Type: ${file.type}`);
+    });
+</script>