implement update_action and change the context menu's upload to update

Well, firstly ``QFIleInfo(xx).fileName()`` is pretty straightforward, doing the task of the c-style function ``getBaseName`` does. While I am fine with creating another function to do this task, it will bloat the whole codebase easily if you insist to do the c++-method-to-c-function convention conversion.
Moreover ``FileUploadTask`` is more low-level class and should do more check to be used by the higher-level class more conveniently (against API changes or whatever some tolerance should be there).