Is it possible to get a handle on a file which is opened by any external app via my application?
Using Cloud-Storage Apps as an example, I would like to track changes to a file opened via the Storage-Provider App, so the manipulated file can be uploaded again afterwards.
There are two possible answers here, depending on what kind of app you're implementing.
For general tracking purposes, you can try using the ContentsChanged event of the StoreFolderQueryResult/StorageFileQueryResult classes within Windows.Storage.Search. That is, you create a file or folder query for what you want to watch, and then register an event handler. Generally speaking, this works well for stuff on the local file system; it's not guaranteed if you're trying to run a query on files/folders whose backing store is elsewhere.
The subject is too detailed to be described here, but you can refer the "File and Folder Queries" in Chapter 11 of my free ebook Programming Windows Store Apps with HTML, CSS, and JavaScript, Second Edition, page 607. Even though I focus on JS as a language, the discussions of WinRT APIs like this are useful when working in any language...plus the ebook is free so there's nothing to lose.
The other mechanism would be useful if you're implementing an app that provides the interface to a cloud storage backend, like the OneDrive app that's part of Windows. In this case you'd want to use the CachedFileUpdater contract. See Appendix D, page 1288, of my aforementioned book.