Safari can only upload files from the Camera Roll.
Unless the site accepts URL's for file upload, there is no way to directly upload a file from the DropBox site. Even then, it (usually) only works if you have the file in the Public folder and have a URL that shows the entire file's path (ending in the filename).
You're best bet is the iCab Mobile browser.
It can upload files to most sites. The file needs to be copied to iCab first. You can do this by choosing the Open In App option from the DropBox app. iCab is a bit more complex than most browsers. I you need further help, just ask.