public string[] overrideFileExtensions { get; }

This Importer is registered as a Override Importer candidate for all files with an extensions included in this list. Casing is ignored. "EXT" is equivalent to "ext" and registers the Default Importer on files with "EXT" or "ext" extensions. All extensions already listed in the fileExtensions property are ignored. Use GetAvailableImporterOverrides to get a list of available Importers for a specific asset. The active Importer for a given asset can be set by (a) selecting the importer from the Importer drop-down menu in the AssetImporter Inspector; or (b) via scripting with the AssetDatabase.SetImporterOverride method.

Note : Composite extensions are considered during the registration of Override Importer candidates. For example, an importer registered with "composite.extension" will not qualify as a candidate for an asset file named "filename.othercomposite.extension". Priority is given to the composite extension with the closest match.