File upload in custom admin controller

Hi
I want to upload an excel file in backend for every category.
I found similar topic in german forum http://forum.oxid-esales.com/showthread.php?t=18649 and followed all the described steps. My excel file name is sucessfully saved in oxcategories_oxexcelfile but not appearing in folder master/category/excel_file. and these three line of code are already in category_main.php


        $oCategory->assign( $aParams);
        $oCategory= oxRegistry::get("oxUtilsFile")->processFiles( $oCategory);
        $oCategory->save();

Don’t know what and where I’m missing. I would really appreciate your help

Is master/category/excel_file writable?
are threre any errors in webserver error log?