index.js

var fs = require('fs'),
    path = require('path'),
    postcss = require('postcss');

function getUrl(value) {
    var reg = /url\((\s*)(['"]?)(.+?)\2(\s*)\)/g,
        match = reg.exec(value),
        url = match[3];
    return url;
}

function replaceFiles(string, opts) {
    file = getUrl(string);
    ext = path.extname(file).replace('.', '');

    if(ext === 'svg') ext = ext + '+xml';

    fileContents = fs.readFileSync(path.join(opts.root, file));
    output = 'data:image/' + ext + ';base64,' + fileContents.toString('base64');

    return string.replace(file, output);
}

function replaceInline(string, opts) {
    output = new Buffer(string).toString('base64');
    if(opts.prepend) output = opts.prepend + output;
    return output;
}

module.exports = postcss.plugin('postcss-base64', function (opts) {
    return function (css, result) {
        opts = opts || {};

        var exts,
            ext,
            search,
            file,
            fileContents,
            output;

        if(!opts.root) {
            opts.root = process.cwd();
        }

        if(opts.excludeAtFontFace === undefined) {
            opts.excludeAtFontFace = true;
        }

        if(opts.extensions) {
            exts = '\\' + opts.extensions.join('|\\');
            search = new RegExp('url\\(.*(' + exts + ').*\\)', 'i');

            css.each(function (node) {
                if(
                    opts.excludeAtFontFace &&
                    node.type === 'atrule' &&
                    node.name === 'font-face'
                ) {
                    // Don't do @font-face rules
                    return;
                };

                if(node.replaceValues) {
                    node.replaceValues(search, function (string) {
                        return replaceFiles(string, opts);
                    });
                }
            });
        }

        if(opts.pattern) {
            if(!opts.pattern instanceof RegExp) {
                throw new Error('Given search pattern is not a (valid) regular expression.');
            }

            search = opts.pattern;

            css.replaceValues(search, function (string) {
                return replaceInline(string, opts);
            });
        }
    };
});