dropEffect should be set when dragging and dropping

[Verthosa]

Hi,
we noticed an issue with the drag and drop of emails coming from for example Outlook. When dropping an email into the upload field, the mail gets removed from the mailbox.

This can be fixed by setting
event.dataTransfer.dropEffect = 'copy';

in the dragenter, dragover, drop events. Unfortunately we have no access to them using ng-flow.js

Maybe this can be a configuration parameter in the future?

[AidasK]

Pull requests are welcome