I am currently evaluating building an app that uses the Windows Store architecture (formerly known as Metro). This app will target not just Windows Phone devices, but also Windows 10 desktop.
One function of the app would be to drag-and-drop files onto the app from File Explorer. An example scenario would be to click-and-hold on a JPEG image and drag it into the app, which would then process the file. This would provide the user with another option instead of going through file dialogs.
I can find plenty of resources about using drag-and-drop inside Windows Store apps, but it seems there's no information about drag-and-drop from a "normal" Windows desktop app, such as File Explorer.
With Windows 10, Store apps are promoted to proper windows, rather than fullscreen apps, so this functionality seems to be a bit more useful in this environment.
This comment seems to suggest that it isn't supported, but is dated two years ago and refers to Windows 8. Is this still the case with Windows 10?
Here is video tutorial and simple example on GitHub.