android - Image upload with MultipartEntityBuilder -

i have following class, don't know, how resolve contenttype. why cannot used writeto() method created entity? cannot write byteoutputstream.

public class imageuploadrequest<t> extends request<t> {      private static final string file_part_name = "file";      private multipartentitybuilder mbuilder = multipartentitybuilder.create();     private final response.listener<t> mlistener;     private final file mimagetoupload;     protected map<string, string> headers;      public imageuploadrequest(string uploadurl, response.errorlistener errorlistener, response.listener<t> listener, file imagefiletoupload){         super(method.post, uploadurl, errorlistener);          mlistener = listener;         mimagetoupload = imagefiletoupload;         //call helper method build multipart entity         buildmultipartentity();     }      @override     public map<string, string> getheaders() throws authfailureerror {         map<string, string> headers = super.getheaders();          if (headers == null || headers.equals(collections.emptymap())) {             headers = new hashmap<string, string>();         }          headers.put("accept", "application/json");          return headers;     }      private void buildmultipartentity(){         mbuilder.addbinarybody(file_part_name, mimagetoupload, contenttype.create("image/jpeg"), mimagetoupload.getname());         mbuilder.setmode(httpmultipartmode.browser_compatible);         mbuilder.setlaxmode().setboundary("xx").setcharset(charset.forname("utf-8"));     }      @override     public string getbodycontenttype(){         string contenttypeheader = mbuilder.build().getcontenttype().getvalue();         return contenttypeheader;     }      @override     public byte[] getbody() throws authfailureerror{         bytearrayoutputstream bos = new bytearrayoutputstream();         try {             mbuilder.build().writeto(bos);         } catch (ioexception e) {             volleylog.e("ioexception writing bytearrayoutputstream bos, building multipart request.");         }          return bos.tobytearray();     }      @override     protected response<t> parsenetworkresponse(networkresponse response) {         t result = null;         return response.success(result, httpheaderparser.parsecacheheaders(response));     }      @override     protected void deliverresponse(t response) {         mlistener.onresponse(response);     } }