<input type="file" multiple/> not working on certain mobile browsers

Question

I have this simple, multiple file selector in a form. It works fine on desktop, and certain mobile browsers. But not in-app WebView things, and not in Opera Mobile. Chrome mobile works, which is used for Opera, and WebView, and I'm pretty confused...

Can anyone shed any light onto this, and maybe knows a way to make it work in all browsers?

Any help much appreciated.