Yeah, again that highlights it's down to the OS and browser support.
Actually if you went back to iOS 5, there'd be no option to upload files at all.
Surprising as it's a modern browser and OS.
There's still no configurable settings, though. We display the same HTML and load the same scripts regardless of device (with some exceptions to things like the rich text editor) and it's just up to the device how it wants to behave with that.
You might want to try it with another browser or ask for device/OS specific support at an appropriate site.