Makes use of Flysystem for fs abstraction and is setup in a manner that lets me easily swap out local files (single-server apps) with remote files (e.g. S3, for multi-server apps) by tweaking the service definitions. The Avatar file is just an example of how I might handle an image. Validation is enforced before this handler is called.

AppBundle\File\AbstractFile.php

<?php

namespace AppBundle\File;

use AppBundle\File\Namer\NamerInterface;
use AppBundle\File\UrlGenerator\FileUrlGeneratorInterface;
use League\Flysystem\Filesystem;

abstract class AbstractFile
{
    /** @var Filesystem */
    protected $filesystem;
    /** @var NamerInterface */
    protected $namer;
    /** @var FileUrlGeneratorInterface */
    protected $urlGenerator;

    public function setFilesystem(Filesystem $filesystem)
    {
        $this->filesystem = $filesystem;
    }

    public function setNamer(NamerInterface $namer)
    {
        $this->namer = $namer;
    }

    public function setUrlGenerator(FileUrlGeneratorInterface $urlGenerator)
    {
        $this->urlGenerator = $urlGenerator;
    }

    public function getPublicUrl($filename)
    {
        return $this->urlGenerator->generate($this->getBasePath().$filename);
    }

    public function getPublicUrlBase()
    {
        return $this->urlGenerator->generate($this->getBasePath());
    }

    protected function getBasePath()
    {
        return;
    }
}

AppBundle\File\Namer\NamerInterface.php

<?php

namespace AppBundle\File\Namer;

use Symfony\Component\HttpFoundation\File\UploadedFile;

interface NamerInterface
{
    public function getName(UploadedFile $file);
}

AppBundle\File\Namer\OriginalRandomNamer.php

<?php

namespace AppBundle\File\Namer;

use RandomLib;
use Symfony\Component\HttpFoundation\File\UploadedFile;

/**
 * This namer keeps the original filename but appends a random string to the end.
 */
class OriginalRandomNamer implements NamerInterface
{
    private $opts = [
        'length' => 6,
    ];

    /**
     * @param int $length of random portion of filename
     */
    public function setLength($length)
    {
        $this->opts['length'] = (int) $length;
    }

    public function getName(UploadedFile $file)
    {
        $factory = new RandomLib\Factory();
        $generator = $factory->getMediumStrengthGenerator();

        return sprintf(
            '%s_%s.%s',
            pathinfo($file->getClientOriginalName(), PATHINFO_FILENAME),
            $generator->generateString($this->opts['length'], RandomLib\Generator::CHAR_ALNUM),
            $file->getClientOriginalExtension()
        );
    }
}

AppBundle\File\Namer\RandomNamer.php

<?php

namespace AppBundle\File\Namer;

use RandomLib;
use Symfony\Component\HttpFoundation\File\UploadedFile;

/**
 * This namer generates a random filename.
 */
class RandomNamer implements NamerInterface
{
    private $opts = [
        'length' => 6,
    ];

    /**
     * @param int $length of random portion of filename
     */
    public function setLength($length)
    {
        $this->opts['length'] = (int) $length;
    }

    public function getName(UploadedFile $file)
    {
        $factory = new RandomLib\Factory();
        $generator = $factory->getMediumStrengthGenerator();

        return sprintf(
            '%s.%s',
            $generator->generateString($this->opts['length'], RandomLib\Generator::CHAR_ALNUM),
            $file->getClientOriginalExtension()
        );
    }
}

AppBundle\File\UrlGenerator\FileUrlGeneratorInterface.php

<?php

namespace AppBundle\File\UrlGenerator;

interface FileUrlGeneratorInterface
{
    public function generate($path);
}

AppBundle\File\UrlGenerator\Local.php

<?php

namespace AppBundle\File\UrlGenerator;

use Symfony\Component\Routing\Generator\UrlGeneratorInterface;

class Local implements FileUrlGeneratorInterface
{
    private $router;

    public function __construct(UrlGeneratorInterface $router)
    {
        $this->router = $router;
    }

    public function generate($path)
    {
        return $this->router->generate('app_file', ['path' => $path]);
    }
}

AppBundle\File\User\Avatar.php

<?php

namespace AppBundle\File\User;

use AppBundle\Entity\User;
use AppBundle\File\AbstractFile;
use Intervention\Image\Constraint;
use Intervention\Image\ImageManager;
use Symfony\Component\HttpFoundation\File\Exception\FileException;
use Symfony\Component\HttpFoundation\File\UploadedFile;

class AvatarFile extends AbstractFile
{
    private $imageManager;

    public function __construct(ImageManager $imageManager)
    {
        $this->imageManager = $imageManager;
    }

    protected function getBasePath()
    {
        return 'user/avatar/';
    }

    public function upload(User $user)
    {
        $file = $user->getAvatarFile();

        if (!$file instanceof UploadedFile) {
            return;
        }

        if (!$file->isValid()) {
            throw new FileException($file->getErrorMessage());
        }

        $name = $user->getId().'-'.$this->namer->getName($file);
        $image = $this->imageManager->make($file->openFile());
        $extension = strtolower($file->getExtension());
        $resizeConstraint = function (Constraint $constraint) {
            $constraint->aspectRatio();
            $constraint->upsize();
        };

        $image->backup();
        $image->fit(400, 400, $resizeConstraint);
        $this->filesystem->write($this->getBasePath().'400/'.$name, $image->encode($extension, 85));
        $image->reset();

        $image->fit(50, 50, $resizeConstraint);
        $this->filesystem->write($this->getBasePath().'50/'.$name, $image->encode($extension, 85));
        $image->reset();
        
        return $name;
    }
}

config.yml

    #
    # Example file service
    #

    file.user.avatar:
        class: AppBundle\File\User\AvatarFile
        parent: file.abstract
        arguments: ['@intervention.image_manager']
        calls:
            - [setNamer, ['@file.namer.random']]

    #
    # Files
    #

    file.abstract:
        class: AppBundle\File\AbstractFile
        abstract: true;
        calls:
            - [setFilesystem, ['@file.system.default']]
            - [setNamer, ['@file.namer.default']]
            - [setUrlGenerator, ['@file.url_generator.default']]

    file.system.default:
        alias: file.system.local

    file.url_generator.default:
        alias: file.url_generator.local

    file.namer.default:
        alias: file.namer.original_random

    file.namer.random:
        class: AppBundle\File\Namer\RandomNamer

    file.namer.original_random:
        class: AppBundle\File\Namer\OriginalRandomNamer

    # Local

    file.system.local:
        class: League\Flysystem\Filesystem
        arguments: ['@file.system_adapter.local']

    file.url_generator.local:
        class: AppBundle\File\UrlGenerator\Local
        arguments: ['@router']

    file.system_adapter.local:
        class: League\Flysystem\Adapter\Local
        arguments: ['%upload_path%']

routing.yml

app_file:
    path: "%upload_folder%/{path}"
    requirements:
        path: .*