Moodle in English: [help]how to use the webservice api:core_files_upload

Hi,i want to use the api:core_files_upload.but when i use it,moodle throw a execption as follow;

<EXCEPTION class="moodle_exception">

<ERRORCODE>nofile</ERRORCODE>

I use a from to send the request ad fellow:
<form action="../webservice/rest/server.php" method="get">
    <input type="hidden" name="wstoken" value="1825cdb79d6197f84f0fea45163ca7ca">
        <input type="hidden" name="wsfunction" value="core_files_upload">
    <input type="hidden" name="contextid" value="2">
    <input type="hidden" name="component" value="user">
    <input type="hidden" name="filearea" value="private">
    <input type="hidden" name="itemid" value="0">
    <input type="hidden" name="filepath" value="/">
    <input type="hidden" name="filename" value="test.jpg">
    <input type="hidden" name="filecontent" value="base64_encode">
    <input type="hidden" name="contextlevel" value="user">
    <input type="submit" value="Submit" >
</form>the test.jpg's path is the same as the html file.If the filepath is false,can you tell me what can I change it to make it work?
 I'm looking forward to your reply!