diff --git a/wp-includes/post.php b/wp-includes/post.php
index 1a91a183ad..b0967dbad8 100644
--- a/wp-includes/post.php
+++ b/wp-includes/post.php
@@ -6870,6 +6870,11 @@ function wp_mime_type_icon( $mime = 0, $preferred_ext = '.png' ) {
 		$icon = wp_cache_get( "mime_type_icon_$mime" );
 	}
 
+	// Check if preferred file format variable is present and is a validly formatted file extension.
+	if ( ! empty( $preferred_ext ) && is_string( $preferred_ext ) && ! str_starts_with( $preferred_ext, '.' ) ) {
+		$preferred_ext = '.' . strtolower( $preferred_ext );
+	}
+
 	$post_id = 0;
 	if ( empty( $icon ) ) {
 		$post_mimes = array();
diff --git a/wp-includes/version.php b/wp-includes/version.php
index 44e2622217..4a07b0f97c 100644
--- a/wp-includes/version.php
+++ b/wp-includes/version.php
@@ -16,7 +16,7 @@
  *
  * @global string $wp_version
  */
-$wp_version = '6.6-alpha-58247';
+$wp_version = '6.6-alpha-58248';
 
 /**
  * Holds the WordPress DB revision, increments when changes are made to the WordPress DB schema.