Send Multipart form-data resttemplate?

I am trying to send the multipart-form data using resttemplate, but it is not working and seems data it is not reaching on server.

    public ResponseEntity<Map> postImage(URI postURL, String base64) {
    HttpHeaders headers = new HttpHeaders();
    headers.add("Authorization", "Basic basicAuthString");
    headers.add("Content-Type","multipart/form-data");
    base64 =  "base64String";

    MultiValueMap<String, Object> form = new LinkedMultiValueMap<>();
    form.add("$myPicture", base64);

    RestTemplate restTemplate = new RestTemplate();
    FormHttpMessageConverter converter = new FormHttpMessageConverter();
   // converter.setCharset(Charset.forName("UTF-8"));
    restTemplate.getMessageConverters().add(converter);

    HttpEntity<MultiValueMap<String, Object>> requestEntity = new HttpEntity<>(form, headers);
    ResponseEntity<Map> responseObject = restTemplate.postForEntity(postURL, requestEntity, Map.class);
    return responseObject;
}

But same API is working as expected using curl.

 curl -i -X POST -H "Content-Type:multipart/form-data" ^
 -H "Authorization: Basic basicAuth" ^
 -F $myPicture = "base64String" ^
http://{{host}}:{{port}}/WebSDK/entity?q=entity={{entityGUID}},Picture=$myPicture

I tried various ways to get it worked but was unable to find out any resolution from this if possible could someone please guide me here...

Thanks in advance !!

To Fix this scenario I was in need to provide my base64 Encoded String as InputStream.

   class MultipartInputStreamFileResource extends InputStreamResource {
    private final String filename;
    MultipartInputStreamFileResource(InputStream inputStream, String filename) {
        super(inputStream);
        this.filename = filename;
    }
    @Override
    public String getFilename() {
        return this.filename;
    }
    @Override
    public long contentLength() throws IOException {
        return -1; // we do not want to generally read the whole stream into memory ...
    }
}

    HttpHeaders headers = new HttpHeaders();
    headers.setContentType(MediaType.MULTIPART_FORM_DATA);
    headers.setContentLength(requestBody.length());

    MultiValueMap<String, Object> body = new LinkedMultiValueMap<>();
    body.add("$myPicture", new MultipartInputStreamFileResource(IOUtils.toInputStream(requestBody), filename));
    HttpEntity<MultiValueMap<String,Object>> requestEntity = new HttpEntity<>(body, headers);

providing this InputStreamResource instance in the request body helped to fix my issue.

Thanks .