Navigate to the 'File Storage' window from the left navigation bar. Select the Contact, then the matter, and select the ellipsis menu.
* The invoice must already exist in the system before an attachment can be added
The 'Contact' and 'Matter' will be auto-populated.
Select the appropriate invoice:
Click 'Attach file' button
When viewing the invoice, click on the 'Edit' pencil
Scroll to the bottom and Drag & Drop the desired file to attach to the invoice