Thanks for the reply Mary. To clarify, I have not made any changes to the mapping and it was importing attachments without error until yesterday. There's no h_contentlocation field in my conf.json so I've no idea how it is mapping to this, we only define the source file path for attachement, no other attachment related config that I can find.
However the recent update to Service Manager (2246) states:
The attachment procedure has been updated in line with the server changes to enhance the storage and security of attachments.