Not able to insert byte[] "image/pdf" file in spring

Question

I am going to insert data into table which also includes image files. The json data I am sending from AJAX is:

The data is send by changing file type into base64 in javascript.

Some entity of my java class are:

Letter.java

public class Letter {

    private BigDecimal letterNo;
    private BigDecimal letterId;
    private String inout;
    private String inoutNo;
    private String inoutDate;
    private String letterIssuedSubBy;
    private String letterFile;
    private String representativeName;
    private String representativeNameEng;
    private BigDecimal selectionId;
    private BigDecimal assessmentNo;
    private String entryBy;
    private String rStatus;

    private byte[] imageFile;
    private String imageTitle;

//i ommited getters and setters
}

ProcessAssessmenttTwo.java which is my Dto class

public class ProcessAnexTwo {

    private String entryBy;
    private String rStatus;

    private BigDecimal selectionId;
    private String inout;
    private String letterReceivedBy;
    private String letterIssuedSubBy;
    private String representativeNameEng;
    private String inoutDate;
    private BigDecimal letterId;
    private BigDecimal assessmentNo;

    private String imageFiles;

    private byte[] imageFile;

    private String imageTitle;
}

The data is successfully coming to the api but while inserting into the database it is showing me error :

 @PostMapping("saveProcessAnexTwo")
        public ResponseEntity saveAnnexTwo(@RequestBody ProcessAnexTwo processAnexTwo,Model model) {

            System.out.println("entered in saveProcessAnex Two func");

            String imgByte=processAnexTwo.getImageFiles();
            System.out.println(imgByte);
            byte[] nnn=imgByte.getBytes();
            byte[] imageFile= Base64.getEncoder().encode(nnn);
            System.out.println(processAnexTwo.getImageTitle());

            Letter letter = new Letter(processAnexTwo.getLetterId(), processAnexTwo.getInout(),
                    processAnexTwo.getInoutDate(),processAnexTwo.getLetterIssuedSubBy(),null,processAnexTwo.getRepresentativeNameEng(),
                    null,processAnexTwo.getSelectionId(),processAnexTwo.getAssessmentNo(), "PCS",
                    null,imageFile,null);


            BigDecimal letterNo = letterService.insertLetterAnnexTwo(letter);
   return null;

        }

The LetterDaoImpl class to insert is:

@Override
    public BigDecimal saveLetterAnnexTwo(Letter letter) {
        try {
            System.out.println("hitted123 here");
            SimpleJdbcCall simpleJdbcCall = new SimpleJdbcCall(jdbcTemplate).withProcedureName("PCPR_ADD_LETTER");
            Map<String, Object> inParamMap = new HashMap<String, Object>();
            System.out.println(letter.getLetterId());
            inParamMap.put("P_LETTER_NO",null);
            inParamMap.put("P_LETTER_ID",letter.getLetterId());
            inParamMap.put("P_INOUT",letter.getInout());
            inParamMap.put("P_INOUT_NO",null);
            inParamMap.put("P_INOUT_DATE",letter.getInoutDate());
            inParamMap.put("P_LETTER_ISSUED_SUB_BY",letter.getLetterIssuedSubBy());
            inParamMap.put("P_LETTER_FILE",letter.getLetterFile());
            inParamMap.put("P_REPRESENTATIVE_NAME",letter.getRepresentativeName());
            inParamMap.put("P_REPRESENTATIVE_NAME_ENG",letter.getRepresentativeNameEng());
            inParamMap.put("P_SELECTION_ID",letter.getSelectionId());
            inParamMap.put("P_ASSESSMENT_NO",0);
            inParamMap.put("P_ENTRY_BY", letter.getEntryBy());
            inParamMap.put("P_R_STATUS",null);
            inParamMap.put("P_IMAGE_FILE",letter.getImageFile());
            inParamMap.put("P_IMAGE_TITLE",letter.getImageTitle());

            SqlParameterSource in = new MapSqlParameterSource(inParamMap);


            BigDecimal letterNO =  (BigDecimal) simpleJdbcCall.execute(in).get("P_LETTER_NO");


            return letterNO;
            } catch(Exception e) {
                e.printStackTrace();
            }
            return null;


    }

The error i am getting is :

SEVERE: Servlet.service() for servlet [dispatcher] in context with path [] threw exception [Request processing failed; nested exception is java.lang.NullPointerException] with root cause java.lang.NullPointerException at java.util.Base64$Decoder.decode(Base64.java:525) at com.pcs.spring.controller.api.FormApiController.saveAnnexTwo(FormApiController.java:222) at sun.reflect.NativeMethodAccessorImpl.invoke0(Native Method) at sun.reflect.NativeMethodAccessorImpl.invoke(NativeMethodAccessorImpl.java:62) at sun.reflect.DelegatingMethodAccessorImpl.invoke(DelegatingMethodAccessorImpl.java:43) at java.lang.reflect.Method.invoke(Method.java:498) at org.springframework.web.method.support.InvocableHandlerMethod.doInvoke(InvocableHandlerMethod.java:205) at org.springframework.web.method.support.InvocableHandlerMethod.invokeForRequest(InvocableHandlerMethod.java:133) at org.springframework.web.servlet.mvc.method.annotation.ServletInvocableHandlerMethod.invokeAndHandle(ServletInvocableHandlerMethod.java:116) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.invokeHandlerMethod(RequestMappingHandlerAdapter.java:827) at org.springframework.web.servlet.mvc.method.annotation.RequestMappingHandlerAdapter.handleInternal(RequestMappingHandlerAdapter.java:738) at org.springframework.web.servlet.mvc.method.AbstractHandlerMethodAdapter.handle(AbstractHandlerMethodAdapter.java:85) at org.springframework.web.servlet.DispatcherServlet.doDispatch(DispatcherServlet.java:963) at org.springframework.web.servlet.DispatcherServlet.doService(DispatcherServlet.java:897) at org.springframework.web.servlet.FrameworkServlet.processRequest(FrameworkServlet.java:970) at org.springframework.web.servlet.FrameworkServlet.doPost(FrameworkServlet.java:872) at javax.servlet.http.HttpServlet.service(HttpServlet.java:647) at org.springframework.web.servlet.FrameworkServlet.service(FrameworkServlet.java:846) at javax.servlet.http.HttpServlet.service(HttpServlet.java:728) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:305) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at org.apache.tomcat.websocket.server.WsFilter.doFilter(WsFilter.java:51) at org.apache.catalina.core.ApplicationFilterChain.internalDoFilter(ApplicationFilterChain.java:243) at org.apache.catalina.core.ApplicationFilterChain.doFilter(ApplicationFilterChain.java:210) at org.apache.catalina.core.StandardWrapperValve.invoke(StandardWrapperValve.java:222) at org.apache.catalina.core.StandardContextValve.invoke(StandardContextValve.java:123) at org.apache.catalina.authenticator.AuthenticatorBase.invoke(AuthenticatorBase.java:502) at org.apache.catalina.core.StandardHostValve.invoke(StandardHostValve.java:171) at org.apache.catalina.valves.ErrorReportValve.invoke(ErrorReportValve.java:100) at org.apache.catalina.valves.AccessLogValve.invoke(AccessLogValve.java:953) at org.apache.catalina.core.StandardEngineValve.invoke(StandardEngineValve.java:118) at org.apache.catalina.connector.CoyoteAdapter.service(CoyoteAdapter.java:408) at org.apache.coyote.http11.AbstractHttp11Processor.process(AbstractHttp11Processor.java:1041) at org.apache.coyote.AbstractProtocol$AbstractConnectionHandler.process(AbstractProtocol.java:603) at org.apache.tomcat.util.net.JIoEndpoint$SocketProcessor.run(JIoEndpoint.java:312) at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1149) at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:624) at java.lang.Thread.run(Thread.java:748)