ZF2 - Allow an empty input file on submit - zend-framework2

Question

I have a form that allows a user to edit his information and upload an image. The image upload is optional but when I submit the form, I always have an error telling me that the image has not been uploaded.
Here is the partial fieldset code:
public function init() {
$this->add([
'name' => 'avatar',
'type' => 'file',
'attributes' => [
'id' => 'avatar',
'class' => 'input-file',
'accept' => 'image/*'
],
'options' => [
'label' => 'avatar',
]
]);
}
This fieldset implements InputFilterProviderInterface, so here is the code:
public function getInputFilterSpecification() {
return [
'avatar' => [
'type' => '\Zend\InputFilter\FileInput',
'allow_empty' => true,
'required' => false,
'validators' => [
[
'name' => 'FileExtension',
'options' => [
'extension' => ['jpg, jpeg, png'],
'message' => 'wrong_type_file'
]
],
[
'name' => 'FileSize',
'options' => [
'max' => '2MB',
'message' => 'file_too_large'
]
]
],
],
];
}
Despite the fact that this field is not required and allow_empty option is true, when I submit the form and debug the value, I still have validation that fails and this as my debug value:
'avatar' =>
array (size=5)
'name' => string '' (length=0)
'type' => string '' (length=0)
'tmp_name' => string '' (length=0)
'error' => int 4
'size' => int 0
Do you know how could I validate my form event when no file is downloaded?
Thanks in advance for your answers!
EDIT 1: Here are the messages returned by the form after submission:
'avatar' =>
array (size=2)
'fileExtensionNotFound' => string 'Extension d'image non admise (.jpg, .jpeg, .png seulement)'
'fileSizeNotFound' => string 'Le fichier est trop volumineux, 2097152 maximum.' (length=48)
EDIT 2: the action in the controller:
$prg = $this->fileprg($form);
if ($prg instanceof Response) {
return $prg;
} elseif (is_array($prg)) {
$data = $form->getData();
if ($form->isValid()) {
if ($data['person']['file']) {
// #TODO upload file
}
if (!$this->personMapper->updatePerson($data)) {
$this->flashMessenger()->addErrorMessage($this->translator()->translate('error_occurs_backup'));
} else {
$this->flashMessenger()->addSuccessMessage($this->translator()->translate('data_saved'));
}
} else {
// #TODO file error management
\var_dump('not valid');
\var_dump($form->getMessages());
}
}