Only display zip in list of allowed extensions if zips are expected

templates/widgets/zipFileField.hamlet

@@ -4,5 +4,5 @@ $maybe exts <- fmap toNullable permittedExtensions
   <br>
   _{MsgUploadModeExtensionRestriction}: 
   <ul .list--inline .list--comma-separated .list--iconless>
-    $forall ext <- zipExtensions <> exts
+    $forall ext <- bool id (mappend zipExtensions) doUnpack exts
       <li style="font-family: monospace">#{ext}