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.air-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.
Update (8/26/08) : The mime type changed for the AIR 1.0 release.
The correct MIME type for an AIR file is:
For example, to set the MIME type for the Apache server, you would add the following line to your Apache configuration file:
AddType application/vnd.adobe.air-application-installer-package+zip .air