diff --git a/tinyfilemanager.php b/tinyfilemanager.php
index bb35f1c..d0fe6db 100644
--- a/tinyfilemanager.php
+++ b/tinyfilemanager.php
@@ -2945,6 +2945,10 @@ function fm_get_file_mimes($extension)
     $fileTypes['php'] = ['application/x-php'];
     $fileTypes['html'] = ['text/html'];
     $fileTypes['txt'] = ['text/plain'];
+    //Unknown mime-types should be 'application/octet-stream'
+    if(empty($fileTypes[$extension])) {
+      $fileTypes[$extension] = ['application/octet-stream'];
+    }
     return $fileTypes[$extension];
 }