diff --git a/src/Mime/MimeTypeDetector.php b/src/Mime/MimeTypeDetector.php
index 08fe075..5ede490 100644
--- a/src/Mime/MimeTypeDetector.php
+++ b/src/Mime/MimeTypeDetector.php
@@ -76,7 +76,7 @@ public function getMimeType(): ?string
         }
     }
 
-    private function getMimeInternally(): ?string
+    private function getMimeInternally(): bool|string
     {
         // Use existing MimeDetector library
         $mimeDetector = new MimeDetector($this->filePath);