A bug, a format you need, or something that should not be published.
Open an issue on the repository. A file that converted wrongly is the most useful thing you can send — attach it if you can share it, and say what you expected instead.
A format we do not convert yet is a request worth making. Several of the ones here started that way.
Every shared document carries a “Report this document” link at the foot of the page it opens. That link is the fastest route: it identifies the document without you having to describe it.
Questions about what is stored, or a request to delete an account and everything in it, go to the same place. Signed in, you can also delete any document yourself — that removes the row and the stored source together.