It is a nice idea, and well implemented.
Of course, it would be better to have a single firmware with the option to choose between several programs to open the file, but it is not available yet.
I couldn't open the only CHM I have on my SD card with the fbreader - I don't know if it is the reader or the file, so I have to try more files.
Don't take this as a criticism, though.
You've done a good job, overall!