I have a website with an upload field, but the input is hidden with display: none; and I have a div for calling this action.
I
Putting the on top of the fake button with position: absolute and opacity: 0 works. You might also need to set the correct z-index and make the input 100% width and height so it catches the click on top of the button.
Source: https://forums.meteor.com/t/webkit-on-ios-ignores-trigger-click-on-file-input/29828