<input type="file" name="photo" id="photo" onchange="checkfile(this);" />



function checkfile(sender) {
    var validExts = new Array(".zip", ".rar", ".pdf", ".jpeg", ".jpg", ".png", ".tif", ".gif", ".JPG");
    var fileExt = sender.value;
    fileExt = fileExt.substring(fileExt.lastIndexOf('.'));
    if (validExts.indexOf(fileExt) < 0) {
      alert("Invalid file selected, valid files are of " +
               validExts.toString() + " types.");
      return false;
    }
    else return true;
}

var uploadField = document.getElementById("file");

uploadField.onchange = function() {
    if(this.files[0].size > 307200){
       alert("File is too big!");
       this.value = "";
    };
};