upload file as a cover letter during review

Is your feature request related to a problem? Please describe.
I am an editor for an OLH journal, and my job is to shepherd articles through to production. I am frustrated that in the interface where authors enter responses to reviewers and editors, Janeway requires that they write letters into two textboxes. However, authors will frequently ignore this prompt and instead upload a file with the comment "please see attached". This means that the content is not sharable and not in the system.

Describe the solution you'd like
Post discussion team, decided to make it possible for author to upload a file at this point. They have to select it as "letter" type (rather than manuscript file). it will need to be sharable with others in the system. The file and/or the field should be shareable with the reviewers without also disclosing the reviews as is currently required. Eg. Second and n round reviewers should see any response letters and there should be an option to share them directly.

Additional context: outcome of UX interview with KPH, then discussed as part of backlog refinement. Aim to get this into 1.8 or before end of year.