code: function stat (archive, name, cb) {
    return maybe(cb, new Promise((resolve, reject) => {
      // run stat operation
      archive.stat(name, (err, st) => {
        if (err) reject(toBeakerError(err, 'stat'))
        else {
          // read download status
          st.downloaded = 0
          if (!archive.key) {
            // fs, not an archive
            st.downloaded = st.blocks
          } else if (st.isFile()) {
            if (archive.content && archive.content.length) {
              st.downloaded = archive.content.downloaded(st.offset, st.offset + st.blocks)
            }
          }
          resolve(st)
        }
      })
    }))
  }
docs: lookup information about a file