Response.BinaryWrite - incorrect name of file opened on a client side

Code:

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01.Response.Buffer = true; 
02.                    Response.AddHeader("Content-Disposition", string.Format("attachment;filename={0}", documentFileName )); 
03.                    Response.ContentType = documentFoundContentType; 
04.                    Response.ContentEncoding  =  System.Text .Encoding .UTF8  ; 
05.                    Response.HeaderEncoding = System.Text .Encoding .UTF8  ; 
06.                    Response.BinaryWrite(ms.ToArray());

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The dilaogbox that offers me to save the file shows incorrect chars if documentFileName has name not in ASCII format. What I'm supposed to do to have the file name displayed correctly in this dialogbox and to have this file opened under the same name in MS Word, Excel whatever else?