In my case, the
fileItem.getOutputStream();
wasn't working. Thus I made it myself using IOUtils,
File file = new File("/path/to/file");
FileItem fileItem = new DiskFileItem("mainFile", Files.probeContentType(file.toPath()), false, file.getName(), (int) file.length(), file.getParentFile());
try {
InputStream input = new FileInputStream(file);
OutputStream os = fileItem.getOutputStream();
IOUtils.copy(input, os);
// Or faster..
// IOUtils.copy(new FileInputStream(file), fileItem.getOutputStream());
} catch (IOException ex) {
// do something.
}
MultipartFile multipartFile = new CommonsMultipartFile(fileItem);