Drag and drop from Electron apps to other applications is broken. One possible way to work around it is by writing native Node modules replacing Electron’s implementation. That way we can support dragging multiple files out of our app with full support for modifier keys. This article describes how to do this for Windows and MacOS.
But be warned, you have to want drag and drop really, really badly to go down this route. It is a lot of work, as it involves use of native Node modules, the Win32 and Cocoa APIs, and the C, C++ and Objective-C languagues. And in the end it’s still not quite perfect. If that’s not enough to deter you, do read on to embark on a journey to the heart of darkness.