Don't show the Share action for non-file messages

This only shares files, if you try it on anything else it crashes
NeoChat.

(cherry picked from commit a90c26f566)

src/timeline/DelegateContextMenu.qml

@@ -393,6 +393,7 @@ KirigamiComponents.ConvergentContextMenu {
         room: root.room
         eventId: root.eventId
         property string filename: Core.StandardPaths.writableLocation(Core.StandardPaths.CacheLocation) + "/" + eventId.replace(":", "_").replace("/", "_").replace("+", "_") + root.room.fileNameToDownload(eventId)
+        visible: root.mimeType.length > 0
     }
 
     Kirigami.Action {