From FCKeditor Docs

QuickUpload

Adds a file in a folder. It generates a POST.
This command is pretty similar to the FileUpload command. It is used by FCKeditor 2.5.

Sample Request

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

connector.php?command=QuickUpload&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, '/ckfinder/userfiles/files/filename.txt', 'filename.txt', '')  ;
</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 , "/full/path/to/the/filename.txt", "filename.txt" ); : no errors found on the upload process.
• OnUploadCompleted( 1, "", "", "Reason" ); : the upload filed because of "Reason". Note: no paths being sent here, because uploading failed.
• OnUploadCompleted( 201 , "/full/path/to/the/filename.txt(1)", "filename.txt(1)" ); : the file has been uploaded successfully, but its name has been changed to "filename(1).ext".

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