Introduce plugin interface for resolving files from drop event

[okonet]

Do you want to request a feature or report a bug?

• I found a bug
• I want to propose a feature

If this is a feature request, what is motivation or use case for changing the behavior?

After thinking and discussing the architecture for supporting drag'n'drop for folders, it seems the best way would be to allow users to provide a function that returns an array of files. This would allow implementing supper for folders via https://github.com/quarklemotion/html5-file-selector by specifying the function.

[stale]

This issue has been automatically marked as stale because it has not had recent activity in the past 60 days. It will be closed if no further activity occurs. If the problem persists please comment here to bump your issue. Thank You - React Dropzone Maintaners