set correct content type

buildkite/bazelci.py

diff --git a/buildkite/bazelci.py b/buildkite/bazelci.py
index b61fdae..f093427 100644
--- a/buildkite/bazelci.py
+++ b/buildkite/bazelci.py
@@ -645,6 +645,7 @@
     json.dump(info, fp)
     fp.close()
     exitcode = execute_command(["gsutil", "-h", "x-goog-if-generation-match:" + expected_generation,
+                                "-h", "Content-Type:application/json",
                                 "cp", "-a", "public-read", fp.name, bazelci_builds_metadata_url(platform)])
     os.unlink(fp.name)
     return exitcode == 0