@Kyatric has yet given you an answer for what the file is used:
[quote:vvslgrx6]Apparently it contains informations about the icons for the game as a web application.
Or again different:
The appmanifest-file manage the handling of Web Apps. Want to run a game on a website the file is not normally necessary. A web app(lication) is not the same as website with your game ...
Even in the case that you can not upload the file Kyatric has also give a answere ...
For more information:
Where now is still your problem?
You could try it without the file, or not?