Multifunctional File Reader: Handle file upload and drag and drop events

This object can handle file upload and drag and drop events.

It can handle events of clicking or dragging and dropping files oh given page elements.

Then handler can either let the user pick files when the user clicks on the elements or process files when the user drops files in the page elements.

A given callback function is with the processed file data either as text or data URL.

Files that do not match given file name extensions may be ignored.