Module weight may need to be larger to for file_force to work

file_get_content_headers also sets the 'Content-Disposition' and - if it acts later than file_force and thinks that the file is an $inline type, it sets the 'Content-Disposition' to 'inline'.

At least in one of our sites, both file.module and file_force.module have a system weight of 0. This meant that file_force attachments were being shown in the browser and not downloaded.

Setting the weight explicitly on file_force solved this problem for us:
UPDATE `system` SET `weight` = '10' WHERE `name` = 'file_force';