Erron on cron: for each with string in getExcludedMimes

Hello

When I run cron on my website with the latest Search API attachment module I get an error, which tells me there is a problem with the getExcludedMimes function in Plugin/search_api/processor/FilesExtrator.phh. In this foreach

      if (!$extensions) {
        $extensions = $this->defaultExcludedExtensions();
      }
      $excluded_mimes = [];
      foreach ($extensions as $extension) {

        $excluded_mimes[] = $this->mimeTypeGuesser->guess('dummy.' . $extension);
      }

After the debugging I found that the function defaultExcludedExtensions builds an array but at the end this happens:

return implode(' ', $excluded);

So it returns a string.
The foreach needs an array, but a string is given.

I've have included a patch.
I hope this can help someone.