clarified '-d @filename' with a tiny example

.\" nroff -man curl.1

.\" Written by Daniel Stenberg

.\"

.TH curl 1 "8 May 2001" "Curl 7.7.3" "Curl Manual"

.TH curl 1 "3 Aug 2001" "Curl 7.8" "Curl Manual"

.SH NAME

curl \- get a URL with FTP, TELNET, LDAP, GOPHER, DICT, FILE, HTTP or

HTTPS syntax.

If you start the data with the letter @, the rest should be a file name to

read the data from, or - if you want curl to read the data from stdin.  The

contents of the file must already be url-encoded. Multiple files can also be

specified.

specified. Posting data from a file named 'foobar' would thus be done with

"--data @foobar".

To post data purely binary, you should instead use the --data-binary option.