To fix it, just override the validateOnNullValue() method like this :

FileUploadField fileUpload = new FileUploadField("fileupload",new MODEL<FileUpload>());

 fileUpload .add(new AbstractValidator() { 

public boolean validateOnNullValue(){

return true;

}

protected VOID onValidate(IValidatable validatable) { 

FileUpload fileUpload = (FileUpload) validatable.getValue();

}

protected String resourceKey() {

return "yourErrorKey";

}

 });

Now, when no file is selected, and submit BUTTON is clicked, validation will be PERFORMED.

To fix it, just override the validateOnNullValue() method like this :

FileUploadField fileUpload = new FileUploadField("fileupload",new Model<FileUpload>());

 fileUpload .add(new AbstractValidator() { 

public boolean validateOnNullValue(){

return true;

}

protected void onValidate(IValidatable validatable) { 

FileUpload fileUpload = (FileUpload) validatable.getValue();

}

protected String resourceKey() {

return "yourErrorKey";

}

 });

Now, when no file is selected, and submit button is clicked, validation will be performed.