Pretty much most classes with "attachment" in the name. And you're almost certainly going to get it wrong AND break all your support and maintenance processes.
Short answer is "do not do this". Disable attachments, and write an add-on to provide a completely new attachments interface.
I have multiple projects that use variations of the same base workflow. The variations depend on the requirements of the project or issue type. The variations mostly come in the form of new statuses ...
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