In particular, what does this "Choose file" button look like? What happens when you press it?

[img=https://dl.dropboxusercontent.com/spa/quq37nq1583x0lf/w560r8in.png]

Are any mobile users able to use the avatar system at all?

This isn't something I'm able to test since I use a basic phone, but it surprises me that smartphones allegedly don't support the most basic <input type="file"> html.