Index: src/wp-includes/class-wp.php
===================================================================
--- src/wp-includes/class-wp.php	(revision 33601)
+++ src/wp-includes/class-wp.php	(working copy)
@@ -366,6 +366,8 @@
 		} elseif ( empty( $this->query_vars['feed'] ) ) {
 			$headers['Content-Type'] = get_option('html_type') . '; charset=' . get_option('blog_charset');
 		} else {
+			$headers['Content-Type'] = feed_content_type() . '; charset=' . get_option('blog_charset');
+
 			// We're showing a feed, so WP is indeed the only thing that last changed
 			if ( !empty($this->query_vars['withcomments'])
 				|| false !== strpos( $this->query_vars['feed'], 'comments-' )