Halt file upload if maximum size is reached. Is that possible?

Hi guys. I'm building a rapidshare-like app, and as such each user has a disk quota. It's possible to prevent a file save if it exceeds the free space, but I searched for an way to do the same _during_ the file upload. Using flash it's apparently possible, but I don't want to use flash.

One way to do it is via Apache configuration.

But, is there any other way to stop the transfer?

I considered attachment_fu, which says in its config: "validates_as_attachment -- This method prevents files outside of the valid range (:min_size to :max_size, or the :size range) from being saved. It does not however, halt the upload of such files. They will be uploaded into memory regardless of size before validation." So, in attachment_fu, this isn't possible.

MAX_FILE_SIZE worked for PHP, but for Rails, apparently it doesn't.

Thanks in advance for any help,

Paulo Pereira