Allow for overriding content-type of html, rss, atom, etc

[9p4]

In the code https://github.com/getgrav/grav/blob/490bdd6ce7aaae315c033ca5af112a778a42455a/system/src/Grav/Common/Utils.php#L800, Grav hardcodes the content type for some extensions. I want to have a different content type returned for atom/rss in order to style it with xslt.

[rhukster]

You can override the mimetype with your own custom plugin.