Angular 20 and fetching files in unit tests

I have an Angular 19 app which I just upgraded to version 20 and now all of my unit tests which fetch files to process are failing with 404 errors. I have changed noting at all in my unit tests or test configuration.

In my karma.config.js file I specify the file pattern I want Karma to be able to serve (karma docs):

files: [
{
pattern: "test/sample-files/**/*.*",
watched: false,
served: true,
included: true,
},
],


I made a unit test helper function for fetching file data:

async function loadTestFileAsRawDataFile(folderPath: string, fileName: string): Promise<IRawDataFile> {
const path = `base/test/sample-files/${folderPath}/${fileName}`;
const response = await fetch(path);

if (response.statusText !== 'OK') {
throw new Error(
`Test file at '${path}' could not be fetched! Is it included in the karma.conf.js files list?`,
);
}

const fileNameParts = Utils.getFileNameParts(fileName);
const dataAsBuffer = await response.arrayBuffer();

return {
name: fileNameParts.name,
ext: fileNameParts.ext,
type: response.type,
dataAsBuffer,
dataAsString: TestUtils.decoder.decode(dataAsBuffer),
};
}


And then I would call that within my unit tests like this:

fit('should properly ACCEPT a plain text file when tested', async () => {
const testFile = await TestUtils.loadTestFileAsRawDataFile('Plain Text', 'empty.txt');
expect(inputConverter.doesInputFileMatchThisType(testFile)).toBeTrue();
});


This is what my file structure looks like, which is unchanged with the upgrade to Angular 20



And when I run my tests (just focusing one here), it's still requesting the URL I would expect, but it's just getting a 404 back now

Why is this happening all of a sudden, and what do I need to change to fix this?

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