Post blob file inside json object as a binary stream with oracle

I have written function send to email using pl/sql. i need to send other data with blob file as json format. the procedure code as below.

create or replace FUNCTION EmailProc (
    p_to           IN VARCHAR2,
    p_subject      IN VARCHAR2,
    p_body         IN VARCHAR2,  
    p_attachement  IN BLOB  DEFAULT NULL
) RETURN NUMBER AS

    req           utl_http.req;
    res           utl_http.resp;
    url           VARCHAR2(4000) := 'http://192.168.168.01:8080/api/Myemails';
    name          VARCHAR2(4000);
    buffer        VARCHAR2(32767);
    content       CLOB:= '{
                                  "to": "'|| p_to ||'",
                                  "subject": "'|| p_subject ||'",
                                  "body": "'|| p_body ||'",                                  
                                  "Attahement": '|| p_attachement ||'                       
                                }';
    emailresult   NUMBER := 0; 
BEGIN
    BEGIN       

        dbms_output.put_line(content);
        req := utl_http.begin_request(url,'POST',' HTTP/1.1');
        utl_http.set_header(req,'user-agent','mozilla/4.0');
        utl_http.set_header(req,'content-type','application/json; charset=UTF-8');
        utl_http.set_header(
            req,
            'Content-Length',
             DBMS_LOB.getlength (content)
        );

        utl_http.set_body_charset(req, 'UTF-8');
        utl_http.WRITE_RAW(r    => req, data => UTL_RAW.CAST_TO_RAW(content));
        res := utl_http.get_response(req);
        utl_http.read_line(res,buffer,true);
        dbms_output.put_line(buffer);

        IF
            buffer = '"000"'
        THEN
            dbms_output.put_line(buffer);
            emailresult := 1;
        END IF;

        utl_http.end_response(res);
        RETURN emailresult;
    EXCEPTION
        WHEN OTHERS THEN
            utl_http.end_response(res);
            dbms_output.put_line('exception block in email function');
            RETURN 0;
    END;
END EmailProc;

Above code i used "Attahement": '|| p_attachement ||' but it is cannot be implemented like this. i need to some advice, how to implement blob file as binary stream inside json format.

My Model include attachment property

 public byte[] Attahement { get; set; }

My Web API as below

public void Myemails(EmailModel emailModel)

i was able to bind all the data except attachment. any one can give me a advice how do i parse attachment inside json object as binary stream?