A dirty hack to move MantisBT attachments to disk by 100 at a time. Might be needed to execute it several times. Useful when there're a lot of attachments out there. Don't forget to raise max_execution_time and memory_limit PHP parameters in php.ini.

move_attachments.php.patch

--- mantisbt-1.3.20/admin/move_attachments.php.orig     2019-09-26 19:42:19.000000000 +0300
+++ mantisbt-1.3.20/admin/move_attachments.php  2020-09-08 15:43:06.000000000 +0300
@@ -175,8 +175,13 @@

        # Process projects list
        foreach( $p_projects as $t_project ) {
+
+       $start = 0;
+       while ($start <= 1500) {
+               error_log($start);
+
                # Retrieve attachments for the project
-               $t_result = db_query( $t_query, array( $t_project ) );
+               $t_result = db_query( $t_query, array( $t_project ), 100, $start );

                # Project upload path
                $t_upload_path = project_get_upload_path( $t_project );
@@ -253,6 +258,9 @@
                        'data'       => $t_data,
                );

+       $start += 100;
+       }
+
        }
        return $t_moved;
 }