What does the @ (at) symbol before a filename mean in a curl command?

Question

I am looking at an example of a curl command that uses the @ symbol before the filename to post data to the server.

curl http://localhost/ --data-binary @file.txt

What does this mean? Does it insert the contents of the file into the command? Is it specific to curl?

Answer 1

It is specific to curl. From man curl:

--data-binary <data>
  (HTTP) This posts data exactly as specified with no extra processing whatsoever.
  If you start the data with the letter @, the rest should be a filename.  Data is
  posted in a similar manner as --data-ascii does, except that newlines are preserved
  and conversions are never done.

  If this option is used several times, the ones following the first will append data
  as described in -d, --data.