Add GetMimeTypeFromFile to Mojo's network service.

mojo/services/network/public/interfaces/network_service.mojom

Index: mojo/services/network/public/interfaces/network_service.mojom
diff --git a/mojo/services/network/public/interfaces/network_service.mojom b/mojo/services/network/public/interfaces/network_service.mojom
index 1f67253fe482b3a0843c4fd22469c7bee09065d9..bb4b93b8e014f8379ff9f48cc95e02d4f8013968 100644
--- a/mojo/services/network/public/interfaces/network_service.mojom
+++ b/mojo/services/network/public/interfaces/network_service.mojom
@@ -69,4 +69,7 @@ interface NetworkService {
                    HttpServerDelegate delegate)
       => (NetworkError result,
           NetAddress? bound_to);
+
+  // Get the mime type (if any) that is associated with the given file.
+  GetMimeTypeFromFile(string file_path) => (string mime_type);
 };