Thanks for the comments/answers. I assumed that the type of file should be set by the "Open as" context menu. But, I now understand to use the file inspector to set the type. I still have a bit of an issue that in one of my projects, the editor aggressively suggests text completion with swift entities.

Xcode 15.4. Sonoma 14.5. I still get the warning (every time). I'm not motivated enough yet to minimise my app In such a way that I could track the specific code where the warning comes from.