Adds explicit mime type to upload_nightly.sh

This changes the script to pass the `--mime-type` option to `s3cmd`, as
described [here](http://s3tools.org/usage).  The MIME type used is
`application/octet-stream`.

Fixes #12153.

etc/ci/upload_nightly.sh

@@ -16,7 +16,8 @@ upload() {
     package_filename="$(basename "${2}")"
     local -r nightly_upload_dir="s3://servo-builds/nightly/${1}"
     local -r package_upload_path="${nightly_upload_dir}/${package_filename}"
-    s3cmd put "${2}" "${package_upload_path}"
+    s3cmd --mime-type="application/octet-stream" \
+          put "${2}" "${package_upload_path}"
     s3cmd cp "${package_upload_path}" "${nightly_upload_dir}/servo-latest.${3}"
 }