In the requirement's right side vertical menu, click the attachments icon . Click on Browse to choose from a file from your computer, or alternatively drag and drop it into the attachments area.
Requirement owners, co-owners and contributors can add attachments. Users will also need to have project permissions to view and edit the requirement to perform this action. Depending on the settings of the user's project permissions, they may be able to view all the project requirements or just a subset. Users added to a specific requirement as a contributor, reviewer or co-owner will be able to view it.