diff -up qtbase-opensource-src-5.9.4/src/corelib/mimetypes/qmimetype.cpp.fdo101667 qtbase-opensource-src-5.9.4/src/corelib/mimetypes/qmimetype.cpp
--- qtbase-opensource-src-5.9.4/src/corelib/mimetypes/qmimetype.cpp.fdo101667	2018-01-16 00:53:43.000000000 -0600
+++ qtbase-opensource-src-5.9.4/src/corelib/mimetypes/qmimetype.cpp	2018-01-28 07:18:09.502534397 -0600
@@ -418,6 +418,8 @@ QStringList QMimeType::suffixes() const
  */
 QString QMimeType::preferredSuffix() const
 {
+    if (isDefault()) // workaround for unwanted *.bin suffix for octet-stream, https://bugs.freedesktop.org/show_bug.cgi?id=101667, fixed upstream in 1.10 
+        return QString();
     const QStringList suffixList = suffixes();
     return suffixList.isEmpty() ? QString() : suffixList.at(0);
 }