diff --git a/api/src/Mail.php b/api/src/Mail.php
index eb0faf8e3f..f6d16a547c 100644
--- a/api/src/Mail.php
+++ b/api/src/Mail.php
@@ -7323,6 +7323,11 @@ class Mail
 				$structure->setTransferEncoding('8bit');
 				$structure->setCharset('utf-8');
 			}
+			// if we have no text body, but only a PDF or an image, set binary, as it's already base64 transfer-encoded
+			if ($structure->getType() === 'application/pdf' || $structure->getPrimaryType() === 'image')
+			{
+				$structure->setTransferEncoding('binary', ['send' => true]);
+			}
 			$mailer->setBasePart($structure);
 			//error_log(__METHOD__.__LINE__.':'.array2string($structure));