Navigate to the 'File Storage' screen via the left navigation menu:
Click the white '+' sign, and 'Upload files'
Drag & Drop the desired file or folder then select the 'Parent folder,' if applicable
Select the 'Parent folder' from your listed contacts.
Click the white '+' sign and select 'Upload files'
Drag and drop desired files or folder. Since you started the process at the contact level, the parent folder does not need to be changed.
Additionally, you can add files from the 'Contacts' or 'Matters' menu screen by clicking the 'Files tab and the '+'sign.