Adding my own transpiled file format to Typescript imports

I just want to import a file format .xyz whose types are not fixed over all instances of the format:

import { Comment, Article, User } from "./Blog.xyz"

But what I get is the following:

TS2307: Cannot find module './Blog.xyz' or its corresponding type declarations.

What I did so far is the following:

• I developed a compiler that translates my file format to Typescript.
• I added a Typescript plugin to get IDE support for my files. Now I am able to autocomplete my types and constants. This was done by adding a ScriptSnapshot when importing my files relatively.
• I assumed that I need a webpack loader to fill the gap during the Typescript transpilation to Javascript. But it seems I was wrong.
• Then, I discovered this link here (How to import custom file types in Typescript). It describes how to add a declaration file to the type roots of the tsconfig.json . But that is not an option for me since my files have custom types, they are unique for each instance.
• I also discovered that you could add own file extensions to require.extensions, but that also did not helped. What am I missing to make the Typescript build or the webpack build working? What is the missing link to avoid the error message above?

There were several mistakes in my approach. For the missing types of Blog.xyz it is sufficient to add a .d.ts file that serves the file extension. Then the error disappears, but the values for this file keep missing.

For the values my approach was to add a custom webpack loader. Here several issues were blocking me:

1. the transpiled code needs to be free of errors… I missed a comma within a list.
2. the import needs to be on a value, not on a type to trigger the inclusion of the transpiled file (but I can also be wrong).
3. the next loader was ts-loader, whose options need to include the transpileOnly flag.

A lot of stations were better error messages would help a lot.

If you are debugging you custom loader, I recommend the inspect-loader and the --stats option of webpack.