is about newbb2?
look this file -> modules\newbb\class\mimetype.php
and add this line:
 "gz" => "application/x-gunzip",  
sample:
 // $Id: mimetype.php,v 1.1.1.9 2004/11/20 15:18:18 phppp Exp  
 
class mimetype { 
    function getType($filename) 
    {  
        // get base name of the filename provided by user 
        $filename = basename($filename);  
        // break file into parts seperated by . 
        $filename = explode('.', $filename);  
        // take the last part of the file to get the file extension 
        $filename = $filename[count($filename)-1];  
        // find mime type 
        return $this->privFindType($filename); 
    }  
 
    function privFindType($ext) 
    {  
        // create mimetypes array 
        $mimetypes = $this->privBuildMimeArray();  
        // return mime type for extension 
        if (isset($mimetypes[$ext])) { 
            return $mimetypes[$ext];  
            // if the extension wasn't found return octet-stream 
        } else if ("*" == $ext) { 
            return "*"; 
        } else { 
            return 'unknown'; 
        }  
    }  
 
    function getExt($minetype, $extention = '') 
    { 
        $mimetypes = $this->privBuildMimeArray(); 
        foreach ($mimetypes as $extent => $mime) { 
            if ($minetype == $mime && $extent == $extention) { 
                $ext = $extent; 
                return $ext; 
            }  
        }  
        return false; 
    }  
 
    function privBuildMimeArray() 
    { 
        return array("ez" => "application/andrew-inset", 
            "gz" => "application/x-gunzip", 
            "hqx" => "application/mac-binhex40", 
            "cpt" => "application/mac-compactpro", 
            "doc" => "application/msword", 
            "bin" => "application/octet-stream", 
            "dms" => "application/octet-stream", 
            "lha" => "application/octet-stream", 
            "lzh" => "application/octet-stream", 
            "exe" => "application/octet-stream", 
            "class" => "application/octet-stream", 
            "so" => "application/octet-stream", 
            "dll" => "application/octet-stream", 
            "oda" => "application/oda", 
            "pdf" => "application/pdf", 
            "ai" => "application/postscript", 
            "eps" => "application/postscript", 
            "ps" => "application/postscript", 
            "smi" => "application/smil", 
            "smil" => "application/smil", 
            "wbxml" => "application/vnd.wap.wbxml", 
            "wmlc" => "application/vnd.wap.wmlc", 
            "wmlsc" => "application/vnd.wap.wmlscriptc", 
            "bcpio" => "application/x-bcpio", 
            "vcd" => "application/x-cdlink", 
            "pgn" => "application/x-chess-pgn", 
            "cpio" => "application/x-cpio", 
            "csh" => "application/x-csh", 
            "dcr" => "application/x-director", 
            "dir" => "application/x-director", 
            "dxr" => "application/x-director", 
            "dvi" => "application/x-dvi", 
            "spl" => "application/x-futuresplash", 
            "gtar" => "application/x-gtar", 
            "hdf" => "application/x-hdf", 
            "js" => "application/x-javascript", 
            "skp" => "application/x-koan", 
            "skd" => "application/x-koan", 
            "skt" => "application/x-koan", 
            "skm" => "application/x-koan", 
            "latex" => "application/x-latex", 
            "nc" => "application/x-netcdf", 
            "cdf" => "application/x-netcdf", 
            "sh" => "application/x-sh", 
            "shar" => "application/x-shar", 
            "swf" => "application/x-shockwave-flash", 
            "sit" => "application/x-stuffit", 
            "sv4cpio" => "application/x-sv4cpio", 
            "sv4crc" => "application/x-sv4crc", 
            "tar" => "application/x-tar", 
            "tcl" => "application/x-tcl", 
            "tex" => "application/x-tex", 
            "texinfo" => "application/x-texinfo", 
            "texi" => "application/x-texinfo", 
            "t" => "application/x-troff", 
            "tr" => "application/x-troff", 
            "roff" => "application/x-troff", 
            "man" => "application/x-troff-man", 
            "me" => "application/x-troff-me", 
            "ms" => "application/x-troff-ms", 
            "ustar" => "application/x-ustar", 
            "src" => "application/x-wais-source", 
            "xhtml" => "application/xhtml+xml", 
            "xht" => "application/xhtml+xml", 
            "zip" => "application/x-zip-compressed", 
            "Zip" => "application/x-zip-compressed", 
 
            "rar" => "application/x-rar-compressed", 
            "ace" => "application/x-ace-compressed", 
            "au" => "audio/basic", 
            "XM" => "audio/fasttracker", 
            "snd" => "audio/basic", 
            "mid" => "audio/midi", 
            "midi" => "audio/midi", 
            "kar" => "audio/midi", 
            "mpga" => "audio/mpeg", 
            "mp2" => "audio/mpeg", 
            "mp3" => "audio/mpeg", 
            "aif" => "audio/x-aiff", 
            "aiff" => "audio/x-aiff", 
            "aifc" => "audio/x-aiff", 
            "m3u" => "audio/x-mpegurl", 
            "ram" => "audio/x-pn-realaudio", 
            "rm" => "audio/x-pn-realaudio", 
            "rpm" => "audio/x-pn-realaudio-plugin", 
            "ra" => "audio/x-realaudio", 
            "wav" => "audio/x-wav", 
            "wax" => "audio/x-windows-media", 
            "pdb" => "chemical/x-pdb", 
            "xyz" => "chemical/x-xyz", 
            "bmp" => "image/bmp", 
            "gif" => "image/gif", 
            "ief" => "image/ief", 
            "jpeg" => "image/pjpeg", 
            "jpeg" => "image/jpeg", 
            "jpg" => "image/jpeg", 
            "jpe" => "image/jpeg", 
            "png" => "image/x-png", 
            "tiff" => "image/tiff", 
            "tif" => "image/tif", 
            "ico" => "image/icon", 
            "djvu" => "image/vnd.djvu", 
            "djv" => "image/vnd.djvu", 
            "wbmp" => "image/vnd.wap.wbmp", 
            "ras" => "image/x-cmu-raster", 
            "pnm" => "image/x-portable-anymap", 
            "pbm" => "image/x-portable-bitmap", 
            "pgm" => "image/x-portable-graymap", 
            "ppm" => "image/x-portable-pixmap", 
            "rgb" => "image/x-rgb", 
            "xbm" => "image/x-xbitmap", 
            "xpm" => "image/x-xpixmap", 
            "xwd" => "image/x-windowdump", 
            "igs" => "model/iges", 
            "iges" => "model/iges", 
            "msh" => "model/mesh", 
            "mesh" => "model/mesh", 
            "silo" => "model/mesh", 
            "wrl" => "model/vrml", 
            "vrml" => "model/vrml", 
            "css" => "text/css", 
            "html" => "text/html", 
            "htm" => "text/html", 
            "asc" => "text/plain", 
            "txt" => "text/plain", 
            "Log" => "text/plain", 
            "log" => "text/plain", 
            "rtx" => "text/richtext", 
            "rtf" => "text/rtf", 
            "sgml" => "text/sgml", 
            "sgm" => "text/sgml", 
            "tsv" => "text/tab-seperated-values", 
            "wml" => "text/vnd.wap.wml", 
            "wmls" => "text/vnd.wap.wmlscript", 
            "etx" => "text/x-setext", 
            "xml" => "text/xml", 
            "xsl" => "text/xml", 
            "mpeg" => "video/mpeg", 
            "mpg" => "video/mpeg", 
            "mpe" => "video/mpeg", 
            "qt" => "video/quicktime", 
            "mov" => "video/quicktime", 
            "mxu" => "video/vnd.mpegurl", 
            "avi" => "video/x-msvideo", 
            "movie" => "video/x-sgi-movie", 
            "php" => "text/php", 
            "php3" => "text/php3", 
            "ice" => "x-conference-xcooltalk", 
            "unknown" => "application/octet-stream" 
            ); 
    } 
} 
 
?>