Hello James,
In Kintone, you can upload files using the Upload File API.
The basic flow of attaching files to Kintone records is you first need to execute the File Upload API.
And then, using the Add Record and the Update Record API, you need to link the file to an attachment field in a record.
FYI, after executing the File Upload API (when a file is uploaded to Kintone), it produces a fileKey.
You will need to use this filekey in the Add Record or Update Record API to upload the file to an Attachment field.
Just executing the Upload File API does not upload the file to an Attachment field of an App.
I hope this helps.