root/devel/mod/file/lib/inline_mimetypes.php

<?php

    // Mime-types for files that will display in the browser as opposed to download

        $data['mimetype:inline'][] = "image/jpeg";
        $data['mimetype:inline'][] = "image/gif";
        $data['mimetype:inline'][] = "image/png";
        $data['mimetype:inline'][] = "image/x-png";

?>