JS controls the upload file type instance code of FileUpload

function CheckFile(obj) { 
  var array = new Array('gif', 'jpeg', 'png', 'jpg'); //The file type that can be uploaded  if ( == '') { 
    alert("Let's choose the picture you want to upload!"); 
    return false; 
  } 
  else { 
    var fileContentType = (/^(.*)(\.)(.{1,8})$/)[3]; //This file type regular is very useful :)    var isExists = false; 
    for (var i in array) { 
      if (() == array[i].toLowerCase()) { 
        isExists = true; 
        return true; 
      } 
    } 
    if (isExists == false) { 
       = null; 
      alert("The uploaded image type is incorrect!"); 
      return false; 
    } 
    return false; 
  } 
} 

<asp:FileUpload ID="FileUpload1" runat="server" onchange="CheckFile(this);"/>