Type of progress to display: either number of bytes uploaded
or downloaded.
con
Connection to send output too. Usually stdout() or
stderr.
Examples
# If file size is known, you get a progress bar:
x <- GET("http://courses.had.co.nz/12-oscon/slides.zip", progress())
# Otherwise you get the number of bytes downloaded:
x <- GET("http://httpbin.org/drip?numbytes=4000&duration=3", progress())
providing 
.