Steps to reproduce the issue:
Login as admin and create 2 users (e.g user1 and user2).
Login as user1.
Add a file on Document List.
Give user2 Contributor permissions to the file
Login to Content Services with the user2.
Navigate to the folder which contains the file.
1) Open the file
2) Click on 'Info' button
3) Write a comment in the text box and press 'Enter'
Currently, with Contributor permission, the user2 is unable to add any comments for the file.
The comment added by User2 is successfully added and saved.