The only app I know of that will let you upload files to most sites is iCab Mobile browser. You have to copy the file to iCab's download folder first, using Open in from another app or perhaps a share link from DropBox. Once there it becomes available when you choose the Chose File button on a site.
However, it does not work with all sites. Websites use a lot of different methods to offer uploads, and iCab is not compatible with all of them. The only way to tell is to try.