As a vendor, you can attach files to your messages in Vendor Discussions as long as the Project Owner has toggled on Vendor Attachments from their end. Vendors will see the following message if they have access to attach files, "[Organization Name] has granted Vendor access to send attachments in this conversation".
Click on the Attach File button at the bottom if you would like to upload a file along with your message. Please note that files must be under 1000MB per file.