IIS appends /web.config at the end of static files

I am trying to debug a 401 error in IIS that happens when serving static files to an anonymous user.

Let's say I'm trying to access https://example.com/sso.css. That file physically resides on D:\website\sso.css.

Running procmon, I noticed that the w3wp.exe tries to read D:\website\sso.css\web.config treating the static resource as a folder instead of a file, getting a "File not found" error from the filesystem.

Why is IIS appending \web.config when trying to open a file on the physical file system?