fix: fix file upload issue (#8)

src/Vditor/Models/Upload.cs

@@ -21,7 +21,7 @@ namespace Vditor.Models
         public string Accept { get; set; }
         public bool Multiple { get; set; } = true;
         public string FieldName { get; set; } = "file[]";
-        public Dictionary<string, string> ExtraData { get; set; }
-        public Dictionary<string, string> Headers { get; set; }
+        public Dictionary<string, string> ExtraData { get; set; } = new Dictionary<string, string>();
+        public Dictionary<string, string> Headers { get; set; } = new Dictionary<string, string>();
     }
 }