====== app.ChooseFile ======
===== app.ChooseFile(prompt, mimetype, callback) =====


Calls a system dialog to allow the user to choose a file

For audio files:-

<code>app.ChooseFile("Choose a file", "audio/*", assets_onFileChoose);</code>

For text files:-

<code>app.ChooseFile("Choose a file", "text/*", onFileChoose);</code>

For image files:-

<code>app.ChooseFile("Choose a file", "image/*", onFileChoose);</code>

For all files:-

    app.ChooseFile("Choose a file", "*/*", onFileChoose);