Providing a custom thumbnail for a Google Drive app.

gistfile1.js

html2canvas(document.getElementById('editor'),
    {
        onrendered: function (canvas) {
            doc.dirty = false;
            // doc.info is the File structure
            var data = angular.extend({}, doc.info);
            if (doc.info.editable) {
                data.content = doc.info.content;
                data.thumbnail =  {
                    // Base 64 URL friendly
                    "image": canvas.toDataURL().split(',')[1].replace(/\+/g, '-').replace(/\//g, '_').replace(/\=+$/, ''),
                    "mimeType": "image/png"
                };
            }
            return defer.resolve(data);
        }
    }
);