Form:
<f:form.upload property="images" id="images" additionalAttributes="{multiple:multiple}" />

Model:
    /**
     * The images of this gallery
     *
     * @var \Doctrine\Common\Collections\Collection<\Foo\Bar\Domain\Model\Image>
     * @ORM\OneToMany(mappedBy="gallery", cascade={"all"}, orphanRemoval=true)
     */
    protected $images;

    /**
     * Returns all content belonging to this page
     *
     * @return \Doctrine\Common\Collections\Collection<\Foo\Bar\Domain\Model\Image> image
     */
    public function getImages() {
        return $this->images;
    }

    /**
     * @param \Doctrine\Common\Collections\Collection $images
     * @return void
     */
    public function setImages(\Doctrine\Common\Collections\Collection $images) {
        $this->images = $images;
    }

    /**
     * @param \Foo\Bar\Domain\Model\Image $image
     * @return void
     */
    public function addImage(\Foo\Bar\Domain\Model\Image$image) {
    $this->images->add($image);
    }

    /**
     * @param \Foo\Bar\Domain\Model\Image $image
     * @return void
     */
    public function removeImage(\Foo\Bar\Domain\Model\Image$image) {
        $this->images->removeElement($image);
    }