Adding Attachment as an Additional Document:
On the inspection page, tap the + New button on the Additional Documents box
Select the additional file you're wanting to attachment
Name the document with the with client-friendly facing name & tap Save Document

In addition to being able to view the document in the Additional Documents box on the client portal your clients will see it in the sidebar of the report when they click into the web view:

Add Attachment as a Report in the Client View:
Tap the Attach button on the Reports box for the inspection
Select the additional file you're wanting to attachment
Name the document with the with client-friendly facing name & tap Save Document

In addition to being able to view this from the side bar of the web-based report, the attachment will show as a report in the client/agent view: