Modified GetMimeType to return binary type if there is no extension.

Rest.Net/MimeTypeExtensions.cs

@@ -10,8 +10,8 @@ namespace MontoyaTech.Rest.Net
     {
         public static string GetMimeType(this string extension)
         {
-            if (extension == null)
-                throw new ArgumentNullException("extension");
+            if (string.IsNullOrWhiteSpace(extension))
+                return "application/octet-stream";
 
             if (extension.StartsWith("."))
                 extension = extension.Substring(1);