Spring Boot File Content Type Validation With Annotation

CustomExceptionControllerAdvice.java

@ControllerAdvice
public class CustomExceptionControllerAdvice {

    @ExceptionHandler(MultipartException.class)
    void handleMultipartException(MultipartException ex,HttpServletResponse response) throws IOException {
        response.sendError(HttpStatus.BAD_REQUEST.value(),"Please select a file");
    }
  
    @ExceptionHandler(ConstraintViolationException.class)
    public void handleConstraintViolationException(ConstraintViolationException ex,HttpServletResponse response) throws IOException {
        response.sendError(HttpStatus.BAD_REQUEST.value());
    }

}

FileController.java

@RestController
@FieldDefaults(level = AccessLevel.PRIVATE,makeFinal=true)
@RequestMapping("/versions/1")
public class FileController {

    
    @PostMapping("/files")
    @ResponseStatus(HttpStatus.OK)
    public String createFile(@Validated @ValidFile @RequestPart("file") MultipartFile file {

        return file.getOriginalFilename()
    }

}

FileValidator.java

public class FileValidator implements ConstraintValidator<ValidFile, MultipartFile> {

    @Override
    public void initialize(ValidFile constraintAnnotation) {

    }

    @Override
    public boolean isValid(MultipartFile multipartFile, ConstraintValidatorContext context) {

        boolean result = true;

        String contentType = multipartFile.getContentType();
        if (!isSupportedContentType(contentType)) {
            result = false;
        }

        return result;
    }

    private boolean isSupportedContentType(String contentType) {
        return contentType.equals("text/xml")
                || contentType.equals("application/pdf")
                || contentType.equals("image/png")
                || contentType.equals("image/jpg")
                || contentType.equals("image/jpeg");
    }
}

ValidFile.java

@Documented
@Target({ElementType.METHOD, ElementType.FIELD, ElementType.ANNOTATION_TYPE, ElementType.CONSTRUCTOR, ElementType.PARAMETER, ElementType.TYPE_USE})
@Retention(RetentionPolicy.RUNTIME)
@Constraint(validatedBy = {FileValidator.class})
public @interface ValidFile {
    String message() default "Only PDF,XML,PNG or JPG images are allowed";

    Class<?>[] groups() default {};

    Class<? extends Payload>[] payload() default {};
}

For validation exceptions e.g. ConstraintViolationException, I use another exception handler. You may have a look at validator package in the following repo:

yildizmy/upload-csv-into-mysql-using-apache-commons-csv

This can still be bypassed because you're only relying on the headers, which can be spoofed. True content validation can only happen when you inspect the uploaded file (the temporary file) before accepting it and moving it to your destination