The Developer Release Notes for AIR 1.5.1 mention the following:
For client browsers to recognize an AIR application when being downloaded, the web server hosting the AIR application needs to map the application/vnd.adobe.air-applicationinstaller-package+zip MIME content type to the “.air” extension. For example, for an Apache web server, add the following to the AddType section:
AddType application/vnd.adobe.air-application-installer-package+zip .air
You can do this in the web server configuration (e.g. httpd.conf or .htaccess for Apache).
Some webservers have an incorrect default value of the MIME type for AIR applications: e.g. Some versions of Apache serve AIR applications with the following MIME type: application/vnd.adobe.apollo-install-package. This MIME type was prevalent when AIR was still in its Beta phase. Unless the MIME type is corrected on servers, there would be a mismatch when a user downloads an AIR application and tries to open it right away.
The following screenshots (Ubuntu Linux 8.10, Firefox 3.0, AIR 1.5.1 installed) highlight the problem encountered by users if the MIME type is not correctly set on the web server.
When clicking on a .air file in the browser, the following dialog is presented. Note the absence of AIR’s icon next to the file’s name (AIRDashboard.air)
The download list shows the download as failed:
and the application installation does not begin:
If the web server is configured with the correct MIME type, the same sequence is as follows:
If you administer a web server, please ensure that the correct MIME type has been added for .air files. If you are a user and encounter the problem highlighted above, do let your web server administrator know.