确保您的脚本标记出现在您的html声明输入和/或包装您的函数后，如下所示：

Make sure your script tag appears after your html declaration of the input and/or wrapt your function, like this:

window.onload=function(){
document.getElementById('confirm').addEventListener('change', checkFile, false);
approveletter.addEventListener('change', checkFile, false);

function checkFile(e) {

    var file_list = e.target.files;

    for (var i = 0, file; file = file_list[i]; i++) {
        var sFileName = file.name;
        var sFileExtension = sFileName.split('.')[sFileName.split('.').length - 1].toLowerCase();
        var iFileSize = file.size;
        var iConvert = (file.size / 10485760).toFixed(2);

        if (!(sFileExtension === "pdf" || sFileExtension === "doc" || sFileExtension === "docx" || sFileExtension === "png") || iFileSize > 10485760) {
            txt = "File type : " + sFileExtension + "\n\n";
            txt += "Size: " + iConvert + " MB \n\n";
            txt += "Please make sure your file is in pdf or doc format and less than 10 MB.\n\n";
            alert(txt);
        }
    }
}
}