code: public static void processInputFileField(MultipartBody request, String fieldName, InputFile inputFile) {
          String fileId = inputFile.getFileID();
          if (fileId != null) {
              request.field(fieldName, fileId, false);
          } else if (inputFile.getInputStream() != null) {
              request.field(fieldName, new InputStreamBody(inputFile.getInputStream(), inputFile.getFileName()), true);
          } else { // assume file is not null (this is existing behaviour as of 1.5.1)
              request.field(fieldName, new FileContainer(inputFile), true);
          }
      }
docs: Adds an input file to a request, with the given field name.
  
  @param request The request to be added to.
  @param fieldName The name of the field.
  @param inputFile The input file.