Well, for Android I’m using the file chooser plugin but for iOS I’ve switched from the “FilePicker” plugin to a simple HTML5 “file” input element, which turns out to be easier to use AND functionally superior … see here how to implement it:
Actually I did not literally use the code from the gist, I still had to make a few changes to get it working properly (with the code from the gist the “onchange” event didn’t always fire). If anyone is interested in my final solution then I can post it here.
And for iOS 9 and higher you need to configure “entitlements” (for iOS 10 therre’s even an additional requirement), all in all I think I spent the better part of a week just to get the iOS solution working!