1. A computer-implemented method, comprising: receiving, by a data processing apparatus in a content management system, expandable content item requests for publishers, each request specifying a publisher, wherein the content management system is separate from the publishers, and at least two requests each specify a different publisher, and for each request: receiving, by the data processing apparatus, data specifying expandable content items to participate in a content item selection process to fulfill the request, the data for at least one of the expandable content item including a location macro, the location macro being a sequence of characters that is mapped to replacement sequences of characters; and determining, by the data processing apparatus, whether the publisher specified by the request utilizes a default location for a publisher side file location or a non-default location for the publisher side file location, wherein: the publisher side file location specifies a location of a publisher side file hosted by the publisher that facilitates processing of an expandable content item served by a vendor in a resource served by the publisher, the vendor being different from the publisher, and wherein each publisher side file for a vendor corresponds to only the expandable content items the vendor serves; the default location is a location for the publisher side filed specified by a vendor of the expandable content item; and the non-default location is a location for the publisher side file that is different from the default location and specified by the publisher; for each request for which the publisher specified by the request utilizes a default location for the publisher side file location, modifying, by the data processing apparatus, the data specifying the at least one expandable content item that includes the location macro to specify the default location by substituting the location macro in the request with data specifying the default location; and for each request for which the publisher specified by the request utilizes a non-default location for the publisher side file location, allowing only the at least one expandable content item that includes the location macro to participate in the content item selection process and modifying, by the data processing apparatus, the data specifying the at least one expandable content item to specify the non-default location by substituting the location macro in the request with data specifying the non-default location.