Adding and Accessing Content That CloudFront Distributes

https://docs.aws.amazon.com/AmazonCloudFront/latest/DeveloperGuide/AddingObjects.html

When you want CloudFront to distribute content (objects), you add files to one of the origins that you specified for the distribution, and you expose a CloudFront link to the files. A CloudFront edge location doesn’t fetch the new files from an origin until the edge location receives viewer requests for them.

CloudFront servers don’t determine the MIME type for the objects that they serve. When you upload a file to your origin, we recommend that you set the Content-Type header field for it.