Stop file upload when filesize too large - jQuery S3

Question

Please view the snippet below. I am using jQuery fileUpload and a direct s3 form to upload videos to AWS. I would like there to be a filesize limit of 5mb which is imposed before the video hits any local server. How can I amend this code so that the upload is halted completely and alerts the user when the filesize is over 5mb and if its under, everything carries on as usual?

I have added a content length limit of 5mb to the S3 form options, so uploads of larger than this are refused, but it would be preferable if there was something to notify the user that an upload is too large to start with.

 $ ->
      $(".direct-upload").each ->
        form = $(this)
        $(this).fileupload
          url: form.attr("action")
          type: "POST"
          autoUpload: true
          dataType: "xml"
          add: (event, data) ->
           if data.files[0].size < 5242880
            $.ajax
              url: "/signed_urls"
              type: "GET"
              dataType: "json"
              data:
                doc:
                  title: data.files[0].name

              async: false
              success: (data) ->
                form.find("input[name=key]").val data.key
                form.find("input[name=policy]").val data.policy
                form.find("input[name=signature]").val data.signature

            data.submit() 

          send: (e, data) ->
            $(".progress, #dropzone").fadeIn()
            $.each data.files, (index, file) ->
                $('.well').html("").append("Uploading: " + file.name + '<br>' + "File size: " + (file.size / 1000000 ) + ' MB')


          progress: (e, data) ->
            percent = undefined
            percent = Math.round((e.loaded / e.total) * 100)
            $(".bar").css "width", percent + "%"

          fail: (e, data) ->
            console.log "fail"

          success: (data) ->
            url = undefined
            url = decodeURIComponent($(data).find("Location").text())
            $("#video_file").val url

            done: (event, data) ->
                    $("#new_video").submit()
                    $(".progress").fadeOut 1200, ->
                      $(".bar").css "width", 0

UPDATE:

This works nicely. Moving the if statement before the /signed_url prevents a file that is too big being sent to the server and alerts the user.

 $ ->
      $(".direct-upload").each (data) ->
        form = $(this)
        $(this).fileupload 
          url: form.attr("action")
          type: "POST"
          autoUpload: true
          dataType: "xml"
          add: (event, data) ->
           if data.files[0].size < 5242880
            $.ajax
              url: "/signed_urls"
              type: "GET"
              dataType: "json"
              data:
                doc:
                  title: data.files[0].name

              async: false
              success: (data) ->
                form.find("input[name=key]").val data.key
                form.find("input[name=policy]").val data.policy
                form.find("input[name=signature]").val data.signature

            data.submit()
           else
             alert("File too big")

          send: (e, data) ->
               $(".progress, #dropzone").fadeIn()
               $.each data.files, (index, file) ->
                  $('.well').html("").append("Uploading: " + file.name + '<br>' + "File size: " + (Math.round(file.size / 1000000 )) + ' MB')


          progress: (e, data) ->
            percent = undefined
            percent = Math.round((e.loaded / e.total) * 100)
            $(".bar").css "width", percent + "%"

          fail: (e, data) ->
            console.log "fail"

          success: (data) ->
            url = undefined
            url = decodeURIComponent($(data).find("Location").text())
            $("#video_file").val url

          done: (event, data) ->
            $("#new_video").submit()
            $(".progress").fadeOut 1200, ->
                $(".bar").css "width", 0

Answer 1

<form method="post" action="./step2.php" enctype="multipart/form-data" id="formID" name="formID">
  <input type="text" name="title" id="title" />
  <input type="file" name="pptfile" id="pptfile"/>
  <input type="submit" name="btn" id="btn" />
 <script type="text/javascript">

    function checkUpload(size)
    {
        if(size>25)
        {
         var n = size.toFixed(2);
            alert('Your file size is: ' + n + "MB, and it is too large to upload! Please try to upload smaller file (25MB or less).");
            document.getElementById("btn").style.display='none';

        }
        else
        {
            //alert('File size is OK');
            $('#tbn').show();
        }
    }
    $('#pptfile').bind('change', function() {
    var fileSize = this.files[0].size/1024/1024;
        checkUpload(fileSize);
    });
</script>
</form>