If you are encountering this error it is because we are checking for file extensions for security reasons, even on remote servers, files with extension .php, .html, .htm & .tmp won’t be allowed download. Starting with version 4.7.75 of the plugin we’ve added a filter,
dlm_check_remote_extension that can bypass the extension check for remote files.
In order to bypass the check you need to add this filter in the child theme’s functions.php:
add_filter( 'dlm_check_remote_extension', '__retun_false' )