OnBuildCache is a TNotifyEvent property that represents the event handler signalled when the MIME type mapping table needs to be loaded from an external or non-standard source. 

When a procedure has been assigned to the OnBuildCache event handler, it is entirely responsible for loading the file extensions and MIME type/subtypes used in the class instance. When OnBuildCache is unassigned, the default behavior is to loaded the MIME type and file extension mappings specific to the development platform. 

OnBuildCache is triggered in the BuildCache method.

BuildCache