The ExternalFileAttributes field of a ZipArchiveEntry is currently hidden within the implementation and not accessible. This has been fine in the Windows world since it's not used but it is used by Unix's zip/unzip programs to store file permissions of an entry. As it currently stands, there is no way to carry Unix RWX permissions within a zip, so at unzip time the unzipper has to guess at permissions and manually chmod the entries. This PR simply exposes the raw field so that it may be used in any circumstance that requires access to the field. Tests are added to ensure that this field is set, read, and roundtrips properly with zips created from other common programs which use the field.