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This is a paid asset, but now you can download UniFileBrowser Free.
UniFileBrowser v2.7
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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.
Integrates cleanly into your project, and works with Unity 3.4 or higher (including Unity 5). Has support for OS X, Windows, and Linux, plus mobile platforms. Includes source code. See the forum topic for news and additional details.