To start with, make sure the input is told it’s about multiple files.
<input type="file" id="files" name="files" multiple>
Then for handeling the data part, does Mendix then receive multiple files or just one?
Might it help to look into the implementation of the dropzone widget for the data handeling?
I see Dojo in the category tags. I would advise you to build a pluggable widget with React, as this is to be the future of widgets :)