Put it higher up in the file:
---#
---# Adding file hash.
ALTER TABLE {$db_prefix}attachments
ADD COLUMN file_hash varchar(40) NOT NULL default '';
---# Altering attachment table.
ALTER TABLE {$db_prefix}attachments
ADD COLUMN fileext varchar(

NOT NULL default '',
ADD COLUMN mime_type varchar(20) NOT NULL default '';
ALTER TABLE {$db_prefix}attachments
ADD COLUMN id_folder tinyint(3) NOT NULL default '1';
---#
---# Populate the attachment extension.
UPDATE {$db_prefix}attachments
SET fileext = LOWER(SUBSTRING(filename, 1 - (INSTR(REVERSE(filename), '.'))))
WHERE fileext = ''
AND INSTR(filename, '.')
AND attachment_type != 3;
Error message changed slightly, see attachment: