Check input[type=file] support

supportInputFile.js

var Util = {};
Util.supportInputFile = function() {
    var input = document.createElement('input'),
        support = false;
    input.type = 'file';
    // ios < 6, 会设置input为disabled
    if (input.type === 'file' && !input.disabled) {
        support = true;
    }
    this.supportInputFile = function() {
        return support;
    };
    return support;
};