Symfony service to subscribe to VichUploaderBundle's POST_UPLOAD event. Saves file details as new entity fields. Note: untested since pasting and editing, may not work as-is.

Attachment.php

<?php

namespace AppBundle\Entity;

use Doctrine\ORM\Mapping as ORM;
use Symfony\Component\HttpFoundation\File\File;
use Symfony\Component\HttpFoundation\File\UploadedFile;
use Symfony\Component\Validator\Constraints as Assert;
use Vich\UploaderBundle\Mapping\Annotation as Vich;

/**
 * @ORM\Table()
 * @ORM\Entity()
 * @Vich\Uploadable()
 * @ORM\HasLifecycleCallbacks()
 */
class Attachment
{
    /**
     * @var File
     *
     * @Assert\File(
     *  maxSize="100Mi",
     *  disallowEmptyMessage=true
     * )
     *
     * @Vich\UploadableField(
     *  mapping="attachment_file",
     *  fileNameProperty="fileName"
     * )
     */
    private $file;

    /**
     * @var string
     *
     * @ORM\Column(type="string", length=255)
     */
    private $fileName;

    /**
     * @var int
     *
     * @ORM\Column(type="integer")
     */
    private $fileSize;

    /**
     * @var string
     *
     * @ORM\Column(type="string", length=255, nullable=true)
     */
    private $mimeType;

    /**
     * @return FileEntity
     */
    public function getFile()
    {
        return $this->file;
    }

    /**
     * @param File|UploadedFile|null $file
     */
    public function setFile(File $file = null)
    {
        $this->file = $file;

        if ($file instanceof UploadedFile) {
            $this->updated = new \DateTime();
        }
    }

    /**
     * @return string
     */
    public function getFileName()
    {
        return $this->fileName;
    }

    /**
     * @param string $fileName
     */
    public function setFileName($fileName)
    {
        $this->fileName = $fileName;
    }

    /**
     * @return int
     */
    public function getFileSize()
    {
        return $this->fileSize;
    }

    /**
     * @param int $fileSize
     */
    public function setFileSize($fileSize)
    {
        $this->fileSize = $fileSize;
    }

    /**
     * @return string
     */
    public function getMimeType()
    {
        return $this->mimeType;
    }

    /**
     * @param string $mimeType
     */
    public function setMimeType($mimeType)
    {
        $this->mimeType = $mimeType;
    }
}

FileSubscriber.php

<?php

namespace AppBundle\Service;

use AppBundle\Entity\Attachment;
use Symfony\Component\EventDispatcher\EventSubscriberInterface;
use Vich\UploaderBundle\Event as VichEvent;

class FileSubscriber implements EventSubscriberInterface
{
    /**
     * @return array
     */
    public static function getSubscribedEvents()
    {
        return [
            VichEvent\Events::POST_UPLOAD => 'postUpload'
        ];
    }

    /**
     * @param VichEvent\Event $event
     */
    public function postUpload(VichEvent\Event $event)
    {
        /** @var Attachment $object */
        $object = $event->getObject();

        $file = $object->getFile();
        $object->setFileSize($file->getSize());
        $object->setMimeType($file->getMimeType());
    }
}

services.yml

services:
  app.file_subscriber:
        class: AppBundle\Service\FileSubscriber
        tags:
            - { name: kernel.event_subscriber }