title: Input type FILE multiple - add files sequentially
body: <p>I am developing a page (one-time use - its registration page) when a visitor opens dialog box and uploads files throught input type file multiple.
  All worked fine, but my client just told me they want to be able to upload multiple files from <strong>different</strong> directories. By opening that dialog more times.</p>
  
  <p>Now, normally what happens is that when I open file select dialog another time, the previously selected files got removed from the input.</p>
  
  <p>Is there any way (pure HTML or JS) that I could add the possibility to "stack" files - add them to the selection (maybe some JS object later converted back to input type file?) so that files can be added to the list of files in the input?</p>