So wrote that you are getting a File exception. Are you  getting something like this?

FileException: Failed to open file 'd:\test.csv' with mode 'r':

If yes, then kindly check if you have the file that you are adding to submit.

Have you tried using a packet sniffer to see what is going on?  This sounds like a server side problem.  Your Curl code looks like the example file upload code you can find at http://www.curl.com/developers/httpforms/file-upload.curl which is a 6.0 applet.  You can look at that applet (or any applets code) by using ctrl+right-click and choosing Edit Applet Source with the Curl IDE.

After changing your example to refer to the server used for the curl dguide example, it worked.

I think that suggests that the request data produced by HttpForm is OK.

{curl 7.0 applet}
{applet {compiler-directives careful? = true}}
 
{let hform:HttpForm  = 
    {new HttpForm,
||--        {url http://www.xyz.com/import.xml?sid=xxxxxxxxxxxxxxxxxxx} ,
        {url http://www.curl.com/cgi-bin/file-upload.pl},
        method = HttpRequestMethod.post,
        encoding = HttpFormData.multipart-mime-type,
        target = _self,
        default-character-encoding = CharEncoding.utf8,
        {CommandButton label = import,
            {on Action at c:CommandButton do
                {c.form.submit}
            }
        }
    }
}
{hform.add-string-param format,csv}
{hform.add-string-param category,Unfiled}
{hform.add-string-param charset,UTF-8}
 
{hform.add-file uploaded-file,
    {url test.csv"}
}
 
{value hform}

The reason I tried directing your example to a different server was to check whether the file content was included.  I assumed that the other form parameters were  sent, but ignored by the server script. However, I did not check that. 

By changing the code to use 'submit-open', so the curl applet gets the response, its possible to use the Curl HTTP Monitor, and that does confirm that all the parameters and the file are included in the request. 

def reply =
   {with-open-streams in = {c.form.submit-open} do
       {in.read-one-string}}
{popup-message {pre {value reply}}}

So there must be something different in what is expected by the script on the Curl server and by your server.  I'm not sure what to suggest, other than comparing the characters sent by the curl form and the http form.  Is that what you meant when asking about 'Content-Disposition' ?

I don't know how the script on the curl server works, so I tried directing it to a php script on localhost that echoes its parameters.  (Sorry, my PHP isn't very good.) That can read the uploaded file.

<?php
header( "Content-type: text/xml" );
function getvar($var){
  return isset($_POST[$var]) ? $_POST[$var] : '';
}
function getfvar($var,$info){
  return isset($_FILES[$var]) ? $_FILES[$var][$info] : '';
}
$format = getvar('format');
$category = getvar('category');
$charset = getvar('charset');
$file_name = getfvar('uploaded-file','name');
$file_type = getfvar('uploaded-file','type');
$file_size = getfvar('uploaded-file','size');
$file_tmp = getfvar('uploaded-file','tmp_name');
$file_text = fread(fopen($file_tmp,'r'),$file_size);
echo <<<QQQ
<?xml version="1.0" encoding="utf-8"?>
<ARGS>
<ARG name='format'>$format</ARG>
<ARG name='category'>$category</ARG>
<ARG name='charset'>$charset</ARG>
<ARG name='file_name'>$file_name</ARG>
<ARG name='file_type'>$file_type</ARG>
<ARG name='file_size'>$file_size</ARG>
<ARG name='file_tmp'>$file_tmp</ARG>
<ARG name='file_text'>$file_text</ARG>
</ARGS>
QQQ;
?>

Which browser and OS are you using for the client?  Isn't there a specific file name associated with the message or status of missing file?  Is that a 404 error?  Or is this a special error page generated by the server which is complaining about the file data being uploaded?  What is the server side software - OS and webserver?