Index: src/wp-includes/media.php
===================================================================
--- src/wp-includes/media.php	(revision 42525)
+++ src/wp-includes/media.php	(working copy)
@@ -1824,7 +1824,17 @@
 		if ( isset( $image_meta['height'], $image_meta['width'] ) ) {
 			$orientation = ( $image_meta['height'] > $image_meta['width'] ) ? 'portrait' : 'landscape';
 		}
-		$output .= "<{$itemtag} class='gallery-item'>";
+		/** 
+		* Filter the gallery item classes. 
+		* 
+		* @since 5.0 
+		* 
+		* @param string $classes    Classes applied to gallery item. 
+		* @param object $attachment Attachment object. 
+		*/ 
+		$classes = 'gallery-item'; 
+		$gallery_item_classes = apply_filters( 'gallery_item_classes', $classes, $attachment ); 
+		$output .= "<{$itemtag} class='{$gallery_item_classes}'>"; 
 		$output .= "
 			<{$icontag} class='gallery-icon {$orientation}'>
 				$image_output