If you are looking for file system to Read / write files just use one of the huge mammoth sized libraries that provide this already, one good library for doing this that is made for c++ is
http://pocoproject.org/ poco was made for c++ as an alternative to boost, as for displaying dialogs just use a library like SFGUI for GUI.
I would highly recommend to not use QT with SFML since you will have to write your own wrapper for QT, and although there is a tutorial from SFML 1.6
http://www.sfml-dev.org/tutorials/1.6/graphics-qt.php, which can be used with sfml 2.3.2 i would just recommend using SFGUI.
SFGUI is a library very tightly integrated with SFML solely for the purpose of GUI, however you will have to build the library yourself with cmake since it does not provide binaries any more, so as long as you can use cmake, with nmake or what ever make file system you are using to build with than this is a very good option.
that being said like every on else here has said there is not point in re inventing the wheel e.g. ( why implement a file system api when there are many others currently out and c++17 is about to have it's own as well ).