diff --git a/src/attachment.hh b/src/attachment.hh
index e9d783e..04dcecc 100644
--- a/src/attachment.hh
+++ b/src/attachment.hh
@@ -19,7 +19,7 @@ namespace pEpMIME
     {
         explicit Attachment(const bloblist_t* b, unsigned nr_in_bloblist, bool has_pEp_msg_attachment)
         : data{b->size ? sv{b->value, b->size} : sv{}}
-        , mime_type{b->mime_type}
+        , mime_type{exists(b->mime_type) ? b->mime_type : "application/octet-stream"}
         , filename{exists(b->filename) ? b->filename : sv{}}
         , dtype{b->disposition}
         , need_te{need_transport_encoding(data)}
@@ -37,7 +37,7 @@ namespace pEpMIME
         
         Attachment(sv _data, sv _mime_type)
         : data{_data}
-        , mime_type{_mime_type}
+        , mime_type{_mime_type.size() ? _mime_type : "application/octet-stream"}
         , filename{}
         , dtype{ PEP_CONTENT_DISP_OTHER }
         , need_te{ need_transport_encoding(data) }