Cómo abrir un archivo con el programa asociado predeterminado

SwingHacks tiene una solución para versiones anteriores de Java.

Creo que usaron el objeto Runtime para ejecutar el comando 'start' en Windows y hay un comando similar en mac.

Aquí tienes:

File myFile = new File("your any type of file url");
FileOpen.openFile(mContext, myFile);

Cree una clase diferente dentro del paquete:

// code to open default application present in the handset


public class FileOpen {

    public static void openFile(Context context, File url) throws IOException {
        // Create URI
        File file=url;
        Uri uri = Uri.fromFile(file);

        Intent intent = new Intent(Intent.ACTION_VIEW);
        // Check what kind of file you are trying to open, by comparing the url with extensions.
        // When the if condition is matched, plugin sets the correct intent (mime) type, 
        // so Android knew what application to use to open the file
        if (url.toString().contains(".doc") || url.toString().contains(".docx")) {
            // Word document
            intent.setDataAndType(uri, "application/msword");
        } else if(url.toString().contains(".pdf")) {
            // PDF file
            intent.setDataAndType(uri, "application/pdf");
        } else if(url.toString().contains(".ppt") || url.toString().contains(".pptx")) {
            // Powerpoint file
            intent.setDataAndType(uri, "application/vnd.ms-powerpoint");
        } else if(url.toString().contains(".xls") || url.toString().contains(".xlsx")) {
            // Excel file
            intent.setDataAndType(uri, "application/vnd.ms-excel");
        } else if(url.toString().contains(".zip") || url.toString().contains(".rar")) {
            // WAV audio file
            intent.setDataAndType(uri, "application/x-wav");
        } else if(url.toString().contains(".rtf")) {
            // RTF file
            intent.setDataAndType(uri, "application/rtf");
        } else if(url.toString().contains(".wav") || url.toString().contains(".mp3")) {
            // WAV audio file
            intent.setDataAndType(uri, "audio/x-wav");
        } else if(url.toString().contains(".gif")) {
            // GIF file
            intent.setDataAndType(uri, "image/gif");
        } else if(url.toString().contains(".jpg") || url.toString().contains(".jpeg") || url.toString().contains(".png")) {
            // JPG file
            intent.setDataAndType(uri, "image/jpeg");
        } else if(url.toString().contains(".txt")) {
            // Text file
            intent.setDataAndType(uri, "text/plain");
        } else if(url.toString().contains(".3gp") || url.toString().contains(".mpg") || url.toString().contains(".mpeg") || url.toString().contains(".mpe") || url.toString().contains(".mp4") || url.toString().contains(".avi")) {
            // Video files
            intent.setDataAndType(uri, "video/*");
        } else {
            //if you want you can also define the intent type for any other file

            //additionally use else clause below, to manage other unknown extensions
            //in this case, Android will show all applications installed on the device
            //so you can choose which application to use
            intent.setDataAndType(uri, "*/*");
        }

        intent.addFlags(Intent.FLAG_ACTIVITY_NEW_TASK); 
        context.startActivity(intent);
    }
}