Download UniFileBrowser FREE Unity. 

UniFileBrowser is a fully functional cross-platform file browser, written using OnGUI code. No need for external plugins…take full control of the file-browsing experience from right within Unity itself!

Support for both opening and saving actions is included, plus full keyboard control. Just make a simple function call, and the complete path of the selected file is sent back to the function you specify. Can be re-skinned as desired, and has lots of options for filtering, file extensions, multi-file selection, folder selection, folder limiting, showing volumes, showing file modification dates, file deletion, and more, so you can tailor the experience to the exact functionality you need.