file transfer: rename duplicate files

Add duplicate number after filename, but before extension, and start
count at 1 instead of 0.

Before, it would be '(0)file.jpg', now it is 'file (1).jpg'.

Change-Id: I0e1f74c172c3c4a66baca8b94302ddf9f42f35d1

src/conversationmodel.cpp

@@ -1965,8 +1965,8 @@ ConversationModelPimpl::slotTransferStatusAwaitingHost(long long dringId, datatr
                 auto wantedFilename = destinationDir + info.displayName;
                 auto duplicate = 0;
                 while (std::ifstream(wantedFilename).good()) {
-                    wantedFilename = destinationDir + "(" + std::to_string(duplicate) + ")" + info.displayName;
                     ++duplicate;
+                    wantedFilename = destinationDir + info.displayName.substr(0, extensionIdx) + " (" + std::to_string(duplicate) + ")" + extension;
                 }
                 acceptTransfer(convId, interactionId, wantedFilename);
             }