upload_file(path, type = NULL)path to file
mime type of path. If not supplied, will be guess by
mime::guess_type() when needed.
citation <- upload_file(system.file("CITATION"))
if (FALSE) { # \dontrun{
POST("http://httpbin.org/post", body = citation)
POST("http://httpbin.org/post", body = list(y = citation))
} # }