How to validate a file in a javascript

[akvarel]

Hello!
I am trying to show the errors that can occur by uploading a file (file extension, size).
I am using an adapted version of Jquery Fiel Upload for Ruby on Rails

I need to validate a file (ending of a file, size of the file) before uploading I want to ask if somebody knows how to do it in a following javascript:

<script type="text/javascript" charset="utf-8">
              $(function () {
                  // Initialize the jQuery File Upload widget:
                  $('#fileupload').fileupload();
                  // 
                  // Load existing files:
                  $.getJSON($('#fileupload').prop('action'), function (files) {
                    var fu = $('#fileupload').data('fileupload'), 
                      template;
                    fu._adjustMaxNumberOfFiles(-files.length);
                    console.log(files);
                    template = fu._renderDownload(files)
                      .appendTo($('#fileupload .files'));
                    // Force reflow:
                    fu._reflow = fu._transition && template.length &&
                      template[0].offsetWidth;
                    template.addClass('in');
                    $('#loading').remove();
                  });
            
              });
        </script>

I am thinking of something like this (the code downstairs) but I need to rewrite the above code to present validation line

`types = /(\.|\/)(gif|jpe?g|png)$/i`

and to have a error-message

`alert("#{file.name} is not a gif, jpeg, or png image file")`

. As I am not god at writing javascripts I am asking for help.

 jQuery ->
      $('#new_painting').fileupload
        dataType: "script"
        add: (e, data) ->
          types = /(\.|\/)(gif|jpe?g|png)$/i
          file = data.files[0]
          if types.test(file.type) || types.test(file.name)
            data.context = $(tmpl("template-upload", file))
            $('#new_painting').append(data.context)
            data.submit()
          else
            alert("#{file.name} is not a gif, jpeg, or png image file")
        progress: (e, data) ->
          if data.context
            progress = parseInt(data.loaded / data.total * 100, 10)
            data.context.find('.bar').css('width', progress + '%')

Many many thanks in advanced