Jekyll setting css mime type incorrectly

My version of Jekyll on Ubuntu Linux is not adding the correct mime type to a .css file in the _site/css/ directory. I found this out because it causes errors when uploading site to AWS S3 bucket and the site won't render properly.

Here is what I've done:

1. created a new jekyll site jekyll new s3upload
2. build the site jekyll build
3. serve the site jekyll serve

and it serves fine, renders correctly in the browser, however the newly created file, main.css in _site/css directory has a mime type set as text/plain. If I then upload it to an amazon web services bucket, to be served it fails because it needs the mime type to be set correctly to text/css.

Is this a bug in Jekyll, or how can I correct it so it creates the new .css file with the correct text/css mime type?

Relative noob here.