From the Apollo Release Notes:

In order for client browsers to recognize an Apollo application when being downloded, the Web Server hosting the Apollo applications needs to map the following mimetype
“application/vnd.adobe.apollo-application-installer-package+zip”
to the Apollo extension “.air”. For example, for an Apache Web Server you will add to the AddType section:
AddType application/vnd.adobe.apollo-application-installer-package+zip .air

Note, you can do this in the web server config, or if using Apache, in the .htaccess file.