Fix the mapped value for isImage

For files this is a boolean but the attachment column expects a TINYINT.

See https://www.woltlab.com/community/thread/312610/

Author: dtdesign

wcfsetup/install/files/lib/data/attachment/Attachment.class.php

@@ -419,7 +419,7 @@ public function __get($name)
             'filename' => $file->filename,
             'filesize' => $file->fileSize,
             'fileType' => $file->mimeType,
-            'isImage' => $file->isImage(),
+            'isImage' => $file->isImage() ? 1 : 0,
             'height' => $file->height ?: 0,
             'width' => $file->width ?: 0,
             'thumbnailType' => $file->getThumbnail('')?->getMimeType() ?: '',