Index: src/wp-includes/class-wp-image-editor-gd.php
===================================================================
--- src/wp-includes/class-wp-image-editor-gd.php	(revision 39602)
+++ src/wp-includes/class-wp-image-editor-gd.php	(working copy)
@@ -432,7 +432,7 @@
 			'file'      => wp_basename( apply_filters( 'image_make_intermediate_size', $filename ) ),
 			'width'     => $this->size['width'],
 			'height'    => $this->size['height'],
-			'mime-type' => $mime_type,
+			'mime_type' => $mime_type,
 		);
 	}
 
Index: src/wp-includes/class-wp-image-editor-imagick.php
===================================================================
--- src/wp-includes/class-wp-image-editor-imagick.php	(revision 39602)
+++ src/wp-includes/class-wp-image-editor-imagick.php	(working copy)
@@ -652,7 +652,7 @@
 			'file'      => wp_basename( apply_filters( 'image_make_intermediate_size', $filename ) ),
 			'width'     => $this->size['width'],
 			'height'    => $this->size['height'],
-			'mime-type' => $mime_type,
+			'mime_type' => $mime_type,
 		);
 	}