for one file:
if($this->request->isPost()) {
    $picture = $this->request->getUploadedFiles()[0];
    // If uploaded
    if (!$picture->getError()) {
        $picture->moveTo('path/to/save/' . $picture->getName());
    }
}
for several files:
if($this->request->isPost()) {
    foreach($this->request->getUploadedFiles() as $picture) {
        if(!$picture->getError()) {
            $picture->moveTo('path/to/save/' . $picture->getName());
        }
    }
}
In template:
<form action="" method="post" enctype="multipart/form-data">
    ...
    <input type="file" name="my_picture">
    ...
    <input type="submit" value="Upload">
</form>