function bufferContents(file, enc, cb) {

        // ignore empty files
        if (file.isNull()) {
            cb();
            return;
        }

        // check stream
        if (file.isStream()) {
            this.emit('error', new Error('Streaming not supported'));
            cb();
            return;
        }

        // check svg
        if (!isSvg(file.contents.toString())) {
            cb();
            return;
        }

        // set first file if not already set
        if (!firstFile) {
            firstFile = file;
        }

        // construct SvgFont instance
        if (!svgFont) {
            var fontName = opts.fontName || path.basename(fileName, '.ttf');
            svgFont = new SvgFont(fontName, opts);
        }

        // add file to SvgFont instance
        svgFont.add(file.relative, file.contents);

        cb();
    }
svgs2ttf fontmin plugin

@param {string} file filename
@param {Object} opts opts
@param {string} opts.fontName font name
@return {Object} stream.Transform instance
@api public