From FCKeditor Docs

FileUpload

Adds a file in a folder. It generates a POST

Sample Request

Upload a file in the root directory of the resource type “Files”.

connector.php?command=FileUpload&type=Files&currentFolder=%2F

The request will be done using POST (instead of GET) and the data will encoded in “multipart/form-data”. Only one field will be posted, named “NewFile”, containing the file uploaded data.

Sample Response

<script type="text/javascript">
 window.parent.OnUploadCompleted( 0, '' ) ;
</script>

This command doesn’t expect XML to be returned. Actually HTML must return with a <script> tag. The content type must be “text/html”. The "OnUploadCompleted" is a JavaScript function that is called to expose the upload result.

The possible values are:

• OnUploadCompleted( 0 ) : no errors found on the upload process.
• OnUploadCompleted( 1, 'Reason' ) : the upload filed because of "Reason".
• OnUploadCompleted( 201, 'FileName(1).ext' ) : the file has been uploaded successfully, but its name has been changed to "FileName(1).ext".
• OnUploadCompleted( 202 ) : invalid file.

Note that uploaded file names may contain non ASC-II chars.