zsh completion issue

`$ exercism submit
`This lists the filenames, but won't complete them.

The output of _complete_help is as follows (below) and shows the "_files" function is missing from the argument-rest completion context.
```
$ exercism submit h
tags in context :completion::complete:exercism::
argument-rest (_arguments _exercism)
all-files (_files _exercism)
tags in context :completion::complete:exercism:argument-rest:
argument-rest (_message _arguments _exercism)
```

The problem seems related to the final '*: :' entry. It adds the arguments-rest context and prevents the later file completion. Removing this line fixes the problem for me. After removing the line the help output is (below) and the completion works.

```
$ exercism submit h
tags in context :completion::complete:exercism::
messages (_message _arguments _exercism)
all-files (_files _exercism)
```

I tested this with a empty .zshrc to be sure it wasn't a conflict with my existing config.