Is your feature request related to a problem? Please describe.
Currently my users have to individually upload files from multiple subfolders, increasing the work and potential for errors.

Describe the solution you'd like
I would like my users to be able to drag a folder into the drop zone and upload all of the files, using the same output API as it does with multiple files. This is supported by react-dropzone, the react component that powers dcc.Upload, as you can see by dragging a folder to the example here in there site: https://react-dropzone.js.org/#section-basic-example.

Describe alternatives you've considered
Rewrapping react-dropzone as new dash component but that creates a maintenance overhead that should not be necessary since it is already the component that powers dcc.Upload. I will also in the meantime have multiple dropzones to upload files, but this is less than ideal for normal usage.

Additional context
This was a request on the previous dcc repo: plotly/dash-core-components#1003