diff --git a/plugins/libkolab/lib/kolab_format.php b/plugins/libkolab/lib/kolab_format.php
index 11a5c4fe..1c5fd128 100644
--- a/plugins/libkolab/lib/kolab_format.php
+++ b/plugins/libkolab/lib/kolab_format.php
@@ -531,8 +531,19 @@ abstract class kolab_format
                 continue;
             $attach = new Attachment;
             $attach->setLabel((string)$attr['name']);
-            $attach->setUri('cid:' . $cid, $attr['mimetype']);
-            $vattach->push($attach);
+            $attach->setUri('cid:' . $cid, $attr['mimetype'] ?: 'application/octet-stream');
+            if ($attach->isValid()) {
+              $vattach->push($attach);
+            }
+            else {
+              rcube::raise_error(array(
+                  'code' => 660,
+                  'type' => 'php',
+                  'file' => __FILE__,
+                  'line' => __LINE__,
+                  'message' => "Invalid attributes for attachment $cid: " . var_export($attr, true),
+              ), true);
+            }
         }
 
         foreach ((array) $object['links'] as $link) {