Allow changing the file extraction format

Feel free to provide a patch.

I've added an MR, but here are some errors with unit tests.

Unfortunately, I don't know how to handle the Solr instance requirement during PHPUnit tests. I'll let you figure it out :)

From our project page and README:

Our test suite includes integration tests that require a real Solr server. This
requirement can't be provided by the drupal.org test infrastructure.
Therefore we leverage travis and github actions for our tests and had to establish a more complex workflow:

1. open an issue on drupal.org as usual
2. upload the patch for being reviewed to that issue on drupal.org as usual
3. fork https://github.com/mkalkbrenner/search_api_solr
4. apply your patch and file a PR on github
5. add a link to the github PR to the drupal.org issue

The PR on github will automatically tested by travis or github actions and the test results will
be reflected in the PR conversation.

Sorry for my ignorance :)

I've added a new PR on github.
Attaching a patch here as well.

Automatically closed - issue fixed for 2 weeks with no activity.