It is not possible to do this on an iPad because each app has its own private file space. So, when you're in Safari, that app doesn't have access to the files stored in the filespace of, say, Pages. As far as I'm aware, this can't even be done in GoodReader, but other Members will hopefully correct me if I'm wrong.
There are some very ugly workarounds (you don't want to know!), but it would be better to do this on a PC if you can.
The iCab Mobile browse has an upload feature. You have to move the file to the iCab app first, either by downloading it or using Open In, and it does not work with all sites. Last time I checked it did not work with this forum. Looks like it will, but always fails at the actual upload.
I don't know of any way to tell what sites it will work with other than to try.
Reading the notice in your screen shot it askes you to fill in a 'https:' URL. That is for a secure server. I see you've entered a non secure server 'http:' address. That may be the problem. Or not.