Form/Handler.php
private function uploadMedia(FormInterface $form, FormConfigurationInterface $configuration)
    {
        $this->attachments = [];
        $mediaIds = [];

        foreach ($configuration->getFileFields() as $field => $collectionId) {
            if (!$form->has($field) || !count($form[$field]->getData())) {
                continue;
            }

            $files = $form[$field]->getData();
            $ids = [];

            if (!is_array($files)) {
                $files = [$files];
            }

            /** @var UploadedFile $file */
            foreach ($files as $file) {
                if (!$file instanceof UploadedFile) {
                    continue;
                }

                $media = $this->mediaManager->save(
                    $file,
                    $this->getMediaData($file, $form, $configuration, $collectionId),
                    null
                );

                // save attachments data for swift message
                $this->attachments[] = $file;
                $ids[] = $media->getId();
            }

            $mediaIds[$field] = $ids;
        }

        return $mediaIds;
    }
private function uploadMedia(FormInterface $form, FormConfigurationInterface $configuration)
    {
        $this->attachments = [];
        $mediaIds = [];

        foreach ($configuration->getFileFields() as $field => $collectionId) {
            if (!$form->has($field) || !count($form[$field]->getData())) {
                continue;
            }

            $files = $form[$field]->getData();
            $ids = [];

            if (!is_array($files)) {
                $files = [$files];
            }

            /** @var UploadedFile $file */
            foreach ($files as $file) {
                if (!$file instanceof UploadedFile) {
                    continue;
                }

                $media = $this->mediaManager->save(
                    $file,
                    $this->getMediaData($file, $form, $configuration, $collectionId),
                    null
                );

                // save attachments data for swift message
                $this->attachments[] = $file;
                $ids[] = $media->getId();
            }

            $mediaIds[$field] = $ids;
        }

        return $mediaIds;
    }
Upload media.

@param FormInterface $form
@param FormConfigurationInterface $configuration

@return array