To check permissions, SSH into your Ubuntu box. Find the location of your deployed app bundle, then take a look inside the public assets directory. In your case it will be something like:
Within that directory, list the file permissions using
[server] /opt/testapp/app/programs/web.browser/app/download > ls -l
-r-xr-xr-x 1 meteoruser root 0 Apr 20 09:20 latest.rar
You just want to make sure your deployed downloadable files are set to be readable by whoever your web service is running as. In the above case for example the listed file is readable by everyone (take a look at Understanding Linux File Permissions if you need more info).
If that all looks okay, then something else is corrupting the download. Have you tried adding another type of binary file into your app for download, like an image or a PDF? If you can download other types of binary files than .rar files, then you might have a content-type issue with your web server. You might need to look into adjusting your mime types via nginx for example.