Ralph, from what I've heard, the easiest way is to put the forum into desktop mode on your mobile - then proceed as if it were a large screen.
We've been working on an update to the "How to Upload Pictures" sticky and this is one of the sticky points. I have an Android tablet and it's easiest to put FF into desktop mode, so I honestly have no idea how to post from the "native" browser.
It might help if you'd mention what mobile or type of mobile you're dealing with - others with similar devices might be able to help.
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