expose() returns a sentinel object, similar in spirit to NULL, that tells
the calling function to return its internal data structure. googledrive
stores a lot of information about the Drive API, MIME types, etc., internally
and then exploits it in helper functions, like drive_mime_type(),
drive_fields(), drive_endpoints(), etc. We use these objects to
provide nice defaults, check input validity, or lookup something cryptic,
like MIME type, based on something friendlier, like a file extension. Pass
expose() to such a function if you want to inspect its internal object, in
its full glory. This is inspired by the waiver() object in ggplot2.