PHP5 on Dreamhost is compiled with register_globals set to off. As such, you either need to "globablize" the $HTTP_POST_FILES variables, or use a "superglobal" instead.
From the zend.com manual on PHP 5 regarding reserved variables:
You might also find the Handlilng file uploads section of the PHP Manual (and links there!), as well as the Dreamhost Wiki article on Register_Globals to be helpful. Good Luck!