LSP client does not set textDocument.languageId for .swift files

I have configured the LSP client for .swift files as follows:

(Language is unset because there is no mime-type for Swift in the list)

However, Qt Creator is sending LSP file open notifications that look like this:
{ "textDocument" : { "languageId" : "", "text" : "import CxxStdlib\nimport dpp\n\n\n@main\nstruct HelloWorld {\n static func main()

\n}\n", "uri" : "file:///home/nils/SwiftLearn/HelloWorld/Sources/HelloWorld/main.swift", "version" : 0 }
}
As you can see, languageId is unset causing the LSP server to fail.

The issue has previously been raised in the LSP's GitHub issues: https://github.com/swiftlang/sourcekit-lsp/issues/2028