[BUGFIX] Fix the attachment reference generation

Classes/Service/MailTemplateService.php

@@ -37,6 +37,7 @@ use TYPO3\CMS\Core\Cache\CacheManager;
 use TYPO3\CMS\Core\Cache\Frontend\FrontendInterface;
 use TYPO3\CMS\Core\Mail\MailMessage;
 use TYPO3\CMS\Core\Resource\FileInterface;
+use TYPO3\CMS\Core\Resource\ResourceFactory;
 use TYPO3\CMS\Core\Utility\ExtensionManagementUtility;
 use TYPO3\CMS\Core\Utility\GeneralUtility;
 use TYPO3\CMS\Extbase\Domain\Model\FileReference;
@@ -640,13 +641,20 @@ class MailTemplateService {
 	 */
 	public function addFileResourceAttachment($file) {
 		if ($file instanceof FileReference) {
-			$this->markers[] = $file;
 			$file = $file->getOriginalResource()->getOriginalFile();
-		} elseif ($file instanceof FileInterface) {
-			$fileReference = $this->objectManager->get(FileReference::class);
-			$fileReference->setOriginalResource($file);
-			$this->markers[] = $fileReference;
 		}
+		$fileReference = $this->objectManager->get(FileReference::class);
+		$resourceFactory = $this->objectManager->get(ResourceFactory::class);
+		$falFileReference = $resourceFactory->createFileReferenceObject(
+			[
+				'uid_local' => $file->getUid(),
+				'uid_foreign' => uniqid('NEW_', TRUE),
+				'uid' => uniqid('NEW_', TRUE),
+				'crop' => NULL,
+			]
+		);
+		$fileReference->setOriginalResource($falFileReference);
+		$this->markers[] = $fileReference;
 		$this->addAttachment($file->getContents(), $file->getName(), $file->getMimeType());
 		return $this;
 	}