file upload

[Ahac Sedušak]

Hi,

the company I work for is using Bugzilla for bugs in its products and
has made some changes to it.
Someone changed post_bug.cgi to make it possible to upload files when
posting bugs and also a script for anonymous posting from forms on
other pages. So if there is a bug on a webpage that the company is
responsible for that the users can enter bugs in a form on that page
and it gets sent to a file called apb.cgi on the Bugzilla server which
is post_bug.cgi without things that we don't need (user settings,
rights, etc).
It's my job to add file upload to that form and script, but I have a
problem:

defined $cgi->upload('data')
|| ThrowUserError("file_not_specified");

I get the error here and I'm not sure where this gets defined. I copied
it from post_bug where it was copied from attachment.cgi. (The guy that
made this is not here anymore).
I have checked apb.cgi and post_bug.cgi and haven't found any
difference that would be important for upload.
Can someone explain how and where this should get defined?
($::FORM{'data'} is not empty so the form must be ok)