Safely convert URI to File

Addressing @jglick's [comment](https://github.com/jenkinsci/tfs-plugin/commit/f4e33a28f622f36a2f725b7ed3692877720a043e#commitcomment-4588234) on f4e33a28f622f36a2f725b7ed3692877720a043e regarding the unsafe conversion of `URI` to `File`.