Attachments, (both internal and for clients) can be added from the proposal settings.
After filling out the required fields in proposal settings, set the show all options toggle to ON:
Scroll down to find file attachments. From here you can drag and drop the attachment from your computer into the dotted area.
Or select click here to upload to open your computer's file manager and select a file.
After the attachment has been added, you can use toggle the on/off switch under visible to clients. Leaving the toggle on will allow your client to download the attachment.
Setting the toggle to off will make this an internal attachment, only visible to you and your team.