Convert template_preprocess_file_link() to OOP

I did a code review today.
Feedback from @berdier and @nicxvan has been addressed.

I didn't noticed any other specific items to validate.
The deprecation test isn't required since there aren't any specific changes as such.

The Needs Review Queue Bot → tested this issue. It no longer applies to Drupal core. Therefore, this issue status is now "Needs work".

This does not mean that the patch necessarily needs to be re-rolled or the MR rebased. Read the Issue Summary, the issue tags and the latest discussion here to determine what needs to be done.

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This was just a reroll so leaving at RTBC