README.md

Assumed folder/file structure:

DistributionPackages/My.FoobarCom/Configuration/Settings.Form.yaml
DistributionPackages/My.FoobarCom/Classes/Form/FormElements/FileUpload.php
DistributionPackages/My.FoobarCom/Classes/Form/FormElements/ImageUpload.php
DistributionPackages/My.FoobarCom/Classes/Form/Validation/FileTypeValidator.php
DistributionPackages/My.FoobarCom/Classes/Form/Validation/ImageTypeValidator.php

The code is based on Neos 4.3 LTS. I assume some changes (regarding type hints) need to be made for Neos 7.

FileTypeValidator.php

<?php

namespace My\FoobarCom\Form\Validation;

/*                                                                        *
 * This script belongs to the TYPO3 Flow package "Neos.Form".            *
 *                                                                        *
 * It is free software; you can redistribute it and/or modify it under    *
 * the terms of the GNU Lesser General Public License, either version 3   *
 * of the License, or (at your option) any later version.                 *
 *                                                                        *
 * The TYPO3 project - inspiring people to share!                         *
 *                                                                        */

use Neos\Flow\Annotations as Flow;
use Neos\Flow\ResourceManagement\ResourceManager;

/**
 * The given $value is valid if it is an \Neos\Flow\ResourceManagement\PersistentResource of the configured resolution
 * Note: a value of NULL or empty string ('') is considered valid
 */
class FileTypeValidator extends \Neos\Flow\Validation\Validator\AbstractValidator
{
    /**
     * @var ResourceManager
     * @Flow\Inject
     */
    protected $resourceManager;

    /**
     * @var array
     */
    protected $supportedOptions = [
        'allowedExtensions' => [[], 'Array of allowed file extensions', 'array', true]
    ];

    /**
     * The given $value is valid if it is an \Neos\Flow\ResourceManagement\PersistentResource of the configured resolution
     * Note: a value of NULL or empty string ('') is considered valid
     *
     * @param \Neos\Flow\ResourceManagement\PersistentResource $resource The resource object that should be validated
     * @return void
     * @api
     */
    protected function isValid($resource)
    {
        if (!$resource instanceof \Neos\Flow\ResourceManagement\PersistentResource) {
            $this->addError('The given value was not a Resource instance.', 1327865587);
            return;
        }
        $fileExtension = $resource->getFileExtension();
        if ($fileExtension === null || $fileExtension === '') {
            $this->addError(
                'Die Datei <strong>"' . $resource->getFilename() . '"</strong> hat keine gültige Endung.',
                1327865808
            );
            // Remove the PersistentResource to clean up
            $this->resourceManager->deleteResource($resource);
            return;
        }
        if (!in_array($fileExtension, $this->options['allowedExtensions'])) {
            $this->addError(
                'Wir können <strong>"' . $resource->getFilename(
                ) . '"</strong> leider nicht verarbeiten.<br />Möglich sind Dateien des Typs: <strong>' . implode(
                    ', ',
                    $this->options['allowedExtensions']
                ) . '</strong>',
                1327865764,
                [$resource->getFileExtension()]
            );
            // Remove the PersistentResource to clean up
            $this->resourceManager->deleteResource($resource);
            return;
        }
    }
}

FileUpload.php

<?php

namespace My\FoobarCom\Form\FormElements;

/*                                                                        *
 * This script belongs to the TYPO3 Flow package "Neos.Form".            *
 *                                                                        *
 * It is free software; you can redistribute it and/or modify it under    *
 * the terms of the GNU Lesser General Public License, either version 3   *
 * of the License, or (at your option) any later version.                 *
 *                                                                        *
 * The TYPO3 project - inspiring people to share!                         *
 *                                                                        */

use Neos\Flow\Annotations as Flow;

/**
 * A generic file upload form element with custom fileTypeValidator
 */
class FileUpload extends \Neos\Form\Core\Model\AbstractFormElement
{
    /**
     * @return void
     */
    public function initializeFormElement()
    {
        $this->setDataType('Neos\Flow\ResourceManagement\PersistentResource');
        $fileTypeValidator = new \My\FoobarCom\Form\Validation\FileTypeValidator(array('allowedExtensions' => $this->properties['allowedExtensions']));
        $this->addValidator($fileTypeValidator);
    }
}

ImageTypeValidator.php

<?php

namespace My\FoobarCom\Form\Validation;

/*
 * This file is part of the Neos.Media package.
 *
 * (c) Contributors of the Neos Project - www.neos.io
 *
 * This package is Open Source Software. For the full copyright and license
 * information, please view the LICENSE file which was distributed with this
 * source code.
 */

use Neos\Flow\Annotations as Flow;
use Neos\Flow\Persistence\PersistenceManagerInterface;
use Neos\Flow\ResourceManagement\ResourceManager;
use Neos\Media\Domain\Model\Image;
use Neos\Media\Domain\Model\ImageInterface;
use Neos\Media\Domain\Repository\ImageRepository;

/**
 * Validator that checks the type of a given image
 *
 * Example:
 * [at]Flow\Validate("$image", type="\Neos\Media\Validator\ImageTypeValidator", options={ "allowedTypes"={"jpeg", "png"} })
 */
class ImageTypeValidator extends \Neos\Media\Validator\ImageTypeValidator
{
    /**
     * @var ImageRepository
     * @Flow\Inject
     */
    protected $imageRepository;

    /**
     * @var ResourceManager
     * @Flow\Inject
     */
    protected $resourceManager;

    /**
     * @var PersistenceManagerInterface
     * @Flow\Inject
     */
    protected $persistenceManager;

    /**
     * The given $value is valid if it is an \Neos\Media\Domain\Model\ImageInterface of the
     * configured type (one of the image/* IANA media subtypes)
     *
     * Note: a value of NULL or empty string ('') is considered valid
     *
     * @param ImageInterface $image The image that should be validated
     * @return void
     * @api
     */
    protected function isValid($image)
    {
        $this->validateOptions();

        if (!$image instanceof Image) {
            $this->addError('The given value was not an Image instance.', 1327947256);
            return;
        }
        $allowedImageTypes = $this->options['allowedTypes'];
        array_walk(
            $allowedImageTypes,
            function (&$value) {
                $value = 'image/' . $value;
            }
        );

        if (!in_array($image->getMediaType(), $allowedImageTypes)) {
            $this->addError('The media type "%s" is not allowed for this image.', 1327947647, [$image->getMediaType()]);
            // Remove the PersistentResource to clean up
            $this->imageRepository->remove($image);
            $this->resourceManager->deleteResource($image->getResource());
        }
    }
}

ImageUpload.php

<?php

namespace My\FoobarCom\Form\FormElements;

/*                                                                        *
 * This script belongs to the TYPO3 Flow package "Neos.Form".            *
 *                                                                        *
 * It is free software; you can redistribute it and/or modify it under    *
 * the terms of the GNU Lesser General Public License, either version 3   *
 * of the License, or (at your option) any later version.                 *
 *                                                                        *
 * The TYPO3 project - inspiring people to share!                         *
 *                                                                        */

use Neos\Form\Core\Runtime\FormRuntime;
use My\FoobarCom\Form\Validation\ImageTypeValidator;

/**
 * A generic file upload form element
 */
class ImageUpload extends \Neos\Form\FormElements\ImageUpload
{
    /**
     * Add ImageTypeValidator just before submitting so that the "allowedTypes" can be changed at runtime
     *
     * @param FormRuntime $formRuntime
     * @param mixed $elementValue
     * @return void
     */
    public function onSubmit(FormRuntime $formRuntime, &$elementValue)
    {
        $imageTypeValidator = new ImageTypeValidator(['allowedTypes' => $this->properties['allowedTypes']]);
        $this->addValidator($imageTypeValidator);
    }
}

Settings.Form.yaml

Neos:
  Form:
    presets:
      default:
        formElementTypes:
          'Neos.Form:FileUpload':
            implementationClassName: My\FoobarCom\Form\FormElements\FileUpload
          'Neos.Form:ImageUpload':
            implementationClassName: My\FoobarCom\Form\FormElements\ImageUpload