title: Rxjs chain to convert observable<File> to observable<string> (base64)
body: <p>I have a working code to convert my File object to base64:</p>
  <pre><code>let reader = new FileReader();
       reader.readAsDataURL(myFile);
       reader.onload = () =&gt; {
           let resultStrOrArrayBuf = reader.result;
           if (!(resultStrOrArrayBuf instanceof ArrayBuffer)) {
              ..do something with resultStrOrArrayBuf
           }
      };
  </code></pre>
  <p>However I do now have to integrate this part to an existing rxjs chain. In the chain I receive the File object and would like to go on with the base64 result of the conversion. However the conversion is done with the help of the <code>onload</code> event. Is there some way to convert this event to a new observable and pass this to the chain?</p>