如何获取图像类型

How can I getting the image type in ajax to allow PNG and JPG only before sending data?

$(document).ready(function() {
$("#sharepostbtnX2").click(function(){

      //--HERE CAN ADD AJAX CODE TO ALLOW PNG AND JPG IMAGES ONLY--

    var formData = new FormData($("#postform")[0]);

    $.ajax({
        url: "postingthepost.php",
        type: 'POST',
        data: formData,
        async: false,
        success: function (response) {
            $(response).hide().prependTo("#responds").fadeIn();
        },
        cache: false,
        contentType: false,
        processData: false
    });

    return false;
});

<input type="file" accept="image/*" id="snap" name="snap" multiple="no" title="Choose a Image file to upload" />

in you javascript

var input = $('#snap')[0];
if (input.files && input.files[0]) {
var oFile = input.files[0];
var oFile = oFile.size;
var oFileType = oFile.type

//Option 1
// check for image type (jpg and png are allowed)
var rFilter = /^(image\/jpeg|image\/png)$/i;
if (! rFilter.test(oFileType)) {
    alert('Please select a valid image file (jpg and png are allowed)');
    return;
}
//Option 2
if (!oFile.fileName.match(/\.(jpg|jpeg|png|gif)$/)){

    //TODO do your validation for image type then process ajax 
 }

}

Tutorial Link JavaScript api doc

try this,

  var formData = new FormData($("#postform")[0]);
  var ext = formData.val().split('.').pop().toLowerCase();
  if($.inArray(ext, ['png','jpg','jpeg']) == -1) {
       alert('InValid Image');
  }else{
       alert('Valid Image');
       //do your AJAX Function..
  }