socialforge/public/javascripts/jq-upload/upload.js

$(function() {
    var $upload_file = $('.upload_file');
    var validateImage = function(file){
      if(!/^image\//.test(file.type)){
        alert($upload_file.attr("data-type-support-message") + $upload_file.attr("data-file-type"));
        return false;
      }
      return true;
    };
    $('.upload_file').fileupload({
        url: '/upload_avatar.json?source_type=' + $('.upload_file').attr('data-source-type') +
            '&source_id=' + $('.upload_file').attr('data-source-id'),
        add: function(e,data){
          if(!validateImage(data.files[0])){
            return false;
          }
          data.submit();
        },
        formData: function(form) {
            var data = form.serializeArray();
            var auth = null;
            for(var key in data){
              if(data[key].name == "authenticity_token"){
                auth = data[key];break; 
              }
            }
            return [auth];
        },
        done: function(e, data) {
            var imgSpan = jQuery('#avatar_image');
            imgSpan.attr({
                "src": data.result.text ? data.result.text() : data.result
            });
        }
    });
});