přišel na to, nastavte obrázek jako binární v modelu
@RequestMapping(value = "/update", method = RequestMethod.POST, consumes = "multipart/form-data")
public ResponseEntity<Payee> update(@RequestPart("payee") @Valid Payee payee, @RequestPart("file") @Valid MultipartFile image) throws IOException
{
// routine to update a payee including image
if (image != null)
payee.setImage(new Binary(BsonBinarySubType.BINARY, image.getBytes()));
Payee result = payeeRepository.save(payee);
return ResponseEntity.ok().body(result);
}