upload(path, type = NULL)

## Not run: 
# image
path <- file.path(Sys.getenv("R_DOC_DIR"), "html/logo.jpg")
(x <- HttpClient$new(url = "https://eu.httpbin.org"))
res <- x$post(path = "post", body = list(y = upload(path)))
res$content

# text file, in a list
file <- upload(system.file("CITATION"))
res <- x$post(path = "post", body = list(y = file))
jsonlite::fromJSON(res$parse("UTF-8"))

# text file, as data
res <- x$post(path = "post", body = file)
jsonlite::fromJSON(res$parse("UTF-8"))

## End(Not run)