drush mmsg <Migration_Name>
Files Source Location
If you specify a source location of (say) '/var/www/d6source/files' for your source files, the migration will look for them in a directory path of '/var/www/d6source/files//sites/capetrials.co.za/files'.
Be sure to copy the source files tree into the correct path (/var/www/d6source/files//sites/capetrials.co.za/files).
Also, make sure the "File" field is enabled in D8.
Destination Path Permissions
If you are running
drush mi capetrials_files --update as a local user (and you should be), temporarily change ownership of the D8 destination files directory to the local user.