File preview broken when browsing some Workshop file areas

When browsing for files via the Server files repository in the Workshop instructauthors, instructreviewers and conclusion file areas, the image previews are broken.

Steps to reproduce

1. Create a new Workshop instance. Upload an image into the "Instructions for submission" or "Instructions for assessment" or "Conclusion" field.
2. Save the form.
3. Edit the workshop again.
4. Into some other field (such as into the Description), attempt to embed the previously embedded image via using the Server files repository.

Expected behaviour: You should be able to browse to the relevant file area and see the image preview.

Actual behaviour: The file preview is broken (see screenshot).

Reason

The reason for this is that in workshop_get_file_info() we declare the itemid should not be used (see the sixth argument of the returned file_info_stored object's constructor at the end of the function). This leads to URL where the itemid is not present at all. But then in workshop_pluginfile() we expect the itemid is always passed (even if we ignore it).