I was just struggling with this yesterday. I was trying to post a photo to a friend's Facebook wall while mobile from my Android device, only to learn that Facebook disables, even when viewing the full site, the ability to attach media to a wall post from a mobile device.
There are many time when browser detection works in my favor, but it's content decisions like these that frustrate me to no end with the mobile web.
You were doing this from a browser and not an app? Some mobile browsers disable the file upload dialog, and there's nothing the site can do to get around it. I know it works like this on iPhone (and it's a huge pain in the ass; you'd think they'd AT LEAST tie in the camera to let you upload photos), but I'm not sure about Android. Are you able to upload files on other sites? Again, I don't have an Android device here, so I'm asking, not telling.
I was specifically using the full site because the Android FB app, on 2.1 anyway, is pretty limited as to what it can do. I'm still eagerly awaiting an update to 2.2 from Samsung for my Captivate that will supposedly happen before the end of this month. I'm not holding my breath.
There are many time when browser detection works in my favor, but it's content decisions like these that frustrate me to no end with the mobile web.