FILE_TYPES_BY_EXTENSION define miss app extension type

I make a tool for automation create project,now i shoud create a share app extension target, but it can't complete because miss product output type(*.appex),like static library product type is *.a.

so can we support this feature?

Locate FILE_TYPES_BY_EXTENSION in the repository and review how existing product output extensions, such as static libraries, are represented. Trace the project-generation path that uses this mapping, then verify that an app extension output is recognized consistently with the existing product types.

Written by the indexing model from the issue text.