Go to Settings the gear icon top right of screen > Under the Admin heading click on Attachments.
Click 'Add New Attachment' the green button top right of screen - a pop up window will display and from here you can upload your file > Save.
Note: Ensure any attachment files are optimised for sending by email. EG compressed to the smallest file size possible. Attachments should be kept below 10Mb in size.
You can now add attachments to the Notes tab on a client.