only check if the key file exists to decide if it is an encrypted file or not.
This solves problems with external storage which doesn't support fseek
check if it is a cached file or a version to resolve the correct path to the file key
Merge branch 'master' into encryption_check_if_file_is_encrypted
Merge pull request #5959 from owncloud/encryption_check_if_file_is_encrypted

only check if the key file exists to decide if it is an encrypted file or not