-Use POST as the method for all HTTP requests and send the specified data
-in the request body. @code{--post-data} sends @var{string} as data,
-whereas @code{--post-file} sends the contents of @var{file}. Other than
-that, they work in exactly the same way.
+Use POST as the method for all HTTP requests and send the specified
+data in the request body. @samp{--post-data} sends @var{string} as
+data, whereas @samp{--post-file} sends the contents of @var{file}.
+Other than that, they work in exactly the same way. In particular,
+they @emph{both} expect content of the form @code{key1=value1&key2=value2},
+with percent-encoding for special characters; the only difference is
+that one expects its content as a command-line paramter and the other
+accepts its content from a file. In particular, @samp{--post-file} is
+@emph{not} for transmitting files as form attachments: those must
+appear as @code{key=value} data (with appropriate percent-coding) just
+like everything else. Wget does not currently support
+@code{multipart/form-data} for transmitting POST data; only
+@code{application/x-www-form-urlencoded}. Only one of
+@samp{--post-data} and @samp{--post-file} should be specified.