Reverse FileReader – Reassamble file from string

I get the file to a string:

const r = new FileReader()
r.readAsText(file, 'UTF-8')
r.onload = (evt) => {
     this.file = evt.target.result
};

After upload a Zip, Text or Image file my string looks like: data:text/plain;base64,VTJGc.....

How can i reverse the readAsText and get a downloadable file?