A bug, a format you need, or something that should not be published.
It is the most useful thing you can send. Attach it to the issue if you can share it, say what you expected instead, and name the browser if it looked right somewhere else. A conversion that is wrong on one file is usually wrong on a shape, and the file is how we find the shape.
A format we do not convert yet is a request worth making. Several of the ones here started as one.
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, and it does not need an account.
Questions about what is stored, or a request to delete an account and everything in it, go to the same issues. Signed in, you can also delete any document yourself — that removes the row and the stored source together.