Only relevant if the extension was built using the BuildContrib: There should be an installer script. installer scripts are important, because they allow you to uninstall the plugin as well as install it, and automatically check dependencies for you. They are also the basis of the forthcoming web-based extensions installer.
Is there a single archive file that is updated with each release?
There should be only one archive attached to the extension topic, named the same as the extension. The archive should be in .zip format; an optional .tgz archive with identical content is useful as well.
Does the archive it contain the appropriate directory layout?
It must contain one of:
lib/TWiki/Plugins/YourPlugin.pm (plugins) or
lib/TWiki/Contrib/YourContrib.pm (all other extensions)
It can contain a directory by the same base name (e.g. lib/TWiki/Plugins/YourPlugin/)
There should be no bin/, if it is a plugin
It must not overwrite any standard TWiki file
It must not contain patches
It can contain a BuildContrib installation script. It should not contain a custom installation script.