Validation during post mapping doesn't call ConstraintValidator's isValid function

I have implemented post mapping in one of controller's:

@PostMapping("/management/upload/")
fun uploadMultipartFile(@FileNotEmpty @RequestParam("file") file: MultipartFile,
                        @Valid @ModelAttribute("form") model: FileUploadForm): ModelAndView {
    print("POST::RECEIVED")
    return ModelAndView("management/upload", "form", FileUploadForm())
}

Unfortunately, the request is received without calling isValid() function, which I have implemented in the validator below. I've tried many permutations of (@FileNotEmpty, @RequestParam(...), @Valid), but neither worked. I have no idea why this validation isn't being called, while it's marked by this annotation.

Here is the form class:

data class FileUploadForm(
    @FileNotEmpty
    val file: MultipartFile? = null
)

Validation annotation:

@Constraint(validatedBy = [FileNotEmptyValidatorForMultipartFile::class])
@Target(AnnotationTarget.FIELD,
        AnnotationTarget.FUNCTION,
        AnnotationTarget.FILE,
        AnnotationTarget.PROPERTY,
        AnnotationTarget.ANNOTATION_CLASS,
        AnnotationTarget.PROPERTY_GETTER, AnnotationTarget.VALUE_PARAMETER)
@Retention(AnnotationRetention.RUNTIME)

annotation class FileNotEmpty (
        val message: String = "{javax.validation.constraints.NotBlank.message}",
        val groups: Array<KClass<out Any>> = [],
        val payload: Array<KClass<out Any>> = []
)

Finally, the validator:

class FileNotEmptyValidatorForMultipartFile : ConstraintValidator<FileNotEmpty, MultipartFile> {

    override fun isValid(value: MultipartFile?, context: ConstraintValidatorContext?): Boolean {
        if (value == null) {
            return false
        }
        return !value.isEmpty || value.name.isNotEmpty()
    }
}

EDIT:

@RequestParam("file") provides me proper value, but at the same time file property in my form model is null. Since I use thymeleaf to create forms and annotations for validation, I need to have proper file value inside my model object.