if we read a .part file we try to get the file info from the real file

if we read a .part file we try to get the file info from the real file.

fix issue discussed here https://github.com/owncloud/core/pull/11659#issuecomment-60232838

This makes sure that we don't try to calculate the unencrypted size from part files during upload.

cc @DeepDiver1975 @jknockaert please review, thanks!
This will cause issues if multiple storages have files with the same path