docs: public RequestBody buildRequestBodyMultipart(Map<String, Object> formParams) {
          MultipartBuilder mpBuilder = new MultipartBuilder().type(MultipartBuilder.FORM);
          for (Entry<String, Object> param : formParams.entrySet()) {
              if (param.getValue() instanceof File) {
                  File file = (File) param.getValue();
                  Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"; filename=\"" + file.getName() + "\"");
                  MediaType mediaType = MediaType.parse(guessContentTypeFromFile(file));
                  mpBuilder.addPart(partHeaders, RequestBody.create(mediaType, file));
              } else {
                  Headers partHeaders = Headers.of("Content-Disposition", "form-data; name=\"" + param.getKey() + "\"");
                  mpBuilder.addPart(partHeaders, RequestBody.create(null, parameterToString(param.getValue())));
              }
          }
          return mpBuilder.build();
      }