Add getFile to FileProcessor

[philiplb]

Hi, nice addition, thank you. A few points:

• Returning the whole file might be a good idea here as there might be implementations of the interface not dealing with handles.
• This leaks abstractions: The implementation details of a file resource are hidden so far. But now, a file handle is returned so the client needs to know this in order to close it in the right way. How would you abstract that fact away?
• As soon as this abstraction problem is solved, unit tests are needed.

Midterm (Next week maybe? Depends on the time available), the FileProcessor interface will be replaced by the Flysystem library. Maybe it would be worthwile to wait for that?

[philiplb]

Flysystem also supports streams, so an integration of it is very likely: https://flysystem.thephpleague.com/api/

[philiplb]

As Flysystem is integrated now, this PR is obsolete. But nontheless, thanks again.