Plugin extensions

The recommended way to extend the jQuery File Upload plugin is by using the extension mechanism of the jQuery UI Widget Factory.
This allows to override default Options (including callback methods) as well as methods of the File Upload widget class.

Example on how to override callbacks and add additional options

The following is an example on how to create a custom File Upload widget that adjusts the deletion URL to include a request authenticity token to protect against CSRF attacks by overriding the destroy callback.
It is a simplified version of the plugin extension that is used for the demo (see Demo Implementation).

$.widget('blueimpUIX.fileupload', $.blueimpUI.fileupload, {

    options: {
        authenticityTokenName: 'request_authenticity_token',
        destroy: function (e, data) {
            var fu = $(this).data('fileupload');
            data.url = data.url &&
                fu._addUrlParams(data.url, fu._getAuthenticityToken());
            $.blueimpUI.fileupload.prototype
                .options.destroy.call(this, e, data);
        }
    },
    
    _addUrlParams: function (url, data) {
        return url + (/\?/.test(url) ? '&' : '?') + $.param(data);
    },
    
    _getAuthenticityToken: function () {
        var name = this.options.authenticityTokenName,
            parts = $.cookie(name).split('|'),
            obj = {};
        obj[name] = parts[0];
        return obj;
    }

});

Example on how to override existing plugin methods

The following example overrides the _renderUploadTemplate and _renderDownloadTemplate methods and provides a simple template mechanism that doesn't require the jQuery Templates plugin:

$.widget('blueimpUIX.fileupload', $.blueimpUI.fileupload, {

    options: {
        errorMessages: {
            maxFileSize: 'File is too big',
            minFileSize: 'File is too small',
            acceptFileTypes: 'Filetype not allowed',
            maxNumberOfFiles: 'Max number of files exceeded'
        }
    },

    _renderUploadTemplate: function (files) {
        var that = this,
            rows = $();
        $.each(files, function (index, file) {
            file = that._uploadTemplateHelper(file);
            var row = $('<tr class="template-upload">' + 
                '<td class="preview"></td>' +
                '<td class="name"></td>' +
                '<td class="size"></td>' +
                (file.error ?
                    '<td class="error" colspan="2"></td>'
                :
                    '<td class="progress"><div></div></td>' +
                    '<td class="start"><button>Start</button></td>'
                ) + 
                '<td class="cancel"><button>Cancel</button></td>' +
                '</tr>');
            row.find('.name').text(file.name);
            row.find('.size').text(file.sizef);
            if (file.error) {
                row.addClass('ui-state-error');
                row.find('.error').text(
                    that.options.errorMessages[file.error] || file.error
                );
            }
            rows = rows.add(row);
        });
        return rows;
    },

    _renderDownloadTemplate: function (files) {
        var that = this,
            rows = $();
        $.each(files, function (index, file) {
            file = that._downloadTemplateHelper(file);
            var row = $('<tr class="template-download">' + 
                (file.error ?
                    '<td></td>' +
                    '<td class="name"></td>' +
                    '<td class="size"></td>' +
                    '<td class="error" colspan="2"></td>'
                :
                    '<td class="preview"></td>' +
                    '<td class="name"><a></a></td>' +
                    '<td class="size"></td>' +
                    '<td colspan="2"></td>'
                ) + 
                '<td class="delete"><button>Delete</button></td>' +
                '</tr>');
            row.find('.size').text(file.sizef);
            if (file.error) {
                row.find('.name').text(file.name);
                row.addClass('ui-state-error');
                row.find('.error').text(
                    that.options.errorMessages[file.error] || file.error
                );
            } else {
                row.find('.name a').text(file.name);
                if (file.thumbnail_url) {
                    row.find('.preview').append('<a><img></a>')
                        .find('img').prop('src', file.thumbnail_url);
                    row.find('a').prop('target', '_blank');
                }
                row.find('a').prop('href', file.url);
                row.find('.delete button')
                    .attr('data-type', file.delete_type)
                    .attr('data-url', file.delete_url);
            }
            rows = rows.add(row);
        });
        return rows;
    }

});