diff --git a/lib/private/Files/Type/Detection.php b/lib/private/Files/Type/Detection.php
index 2801bfadd17..ebd28e7cd24 100644
--- a/lib/private/Files/Type/Detection.php
+++ b/lib/private/Files/Type/Detection.php
@@ -181,8 +181,11 @@ class Detection implements IMimeTypeDetector {
 			$fileName = preg_replace('!((\.v\d+)|((\.ocTransferId\d+)?\.part))$!', '', $fileName);
 
 			//try to guess the type by the file extension
-			$extension = strtolower(strrchr($fileName, '.'));
-			$extension = substr($extension, 1); //remove leading .
+			$extension = strrchr($fileName, '.');
+			if ($extension !== false) {
+				$extension = strtolower($extension);
+				$extension = substr($extension, 1); //remove leading .
+			}
 			return (isset($this->mimetypes[$extension]) && isset($this->mimetypes[$extension][0]))
 				? $this->mimetypes[$extension][0]
 				: 'application/octet-stream';