One of the ways Mac OS X determines if a package is an application is through the use of file extensions. The rules to determine if a package is an application are:
On all file systems: the presence of a .app suffix
On HFS+ only: the .app suffix is optional, if the package bit is set and the folder contains a new style info.xml.
Last updated: 2009-06-25
Mac OS X Reference Library Data Management: File Management Workspace Services Programming Topics