Error: gzip: stdin: not in gzip format

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Hey,
I have been trying to mock the fetch API and a function that downloads files. But while mocking the function that downloads files, I am getting the following error:

Command failed with exit code 2: tar xvf joplin-plugin-rich-markdown-0.9.0.tgz

    gzip: stdin: not in gzip format
    tar: Child returned status 1
    tar: Error is not recoverable: exiting now

      at makeError (node_modules/execa/lib/error.js:59:11)
      at handlePromise (node_modules/execa/index.js:114:26)

I have tried fixing it by changing the mock tgz and making sure tgz is valid, but it's not working.
If anyone has suggestions to fix this, please let me know. Here's the PR.