PodClient.upload_file*() methods should return True for POD's "409: File exists" response.
The file is already loaded and can now be referenced by another file item.
This MR justifies itself because of 'file exists' response blocking PodClient.create_photo() method, preventing creation of new photos referencing the same file.
Alternatively, create_photo and create_photo_file methods can be edited to handle the cases where two photos reference the same file.