Add value to file mock

What does this MR do and why?

While testing our svg file imports for another task, I found our file mocks are not very valuable as the simply return the empty string. It would be better to provide a simple return value to at least ensure the test checks that the results are non-empty.

Screenshots or screen recordings

NA

How to set up and validate locally

Pipelines should be green.

MR acceptance checklist

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