Not just outputs, but also inputs. As a hypothetical example, if I had a project that processed MP3 files, and it had bugs on certain files, it would be nice to be able to attach the MP3 files that triggered the bugs to an issue.
Would it not be useful to have test cases covering the broken behaviour (making it not specific to the issue, but rather stuff you could add in a commit)?
We're talking about issues reporting here, if there's a bug with a file being able to include the file itself (by the original reporter or somebody else) is valuable.