Associations when importing multiple files from the API
@Jaycee_Lewis - Can you see if you can get an update on this? I am also having similar issues. I have 2 CSV files(Company, Custom Object). The Company file contains (RecordId, a custom unique property), and the second file contains all the Custom Objects, including a column for matching. The API documentation seems quite limited on handling multiple files with associations.
So @Otaka I think your JSON needs to be updated by mentioning the idColumnType as HUBSPOT_ALTERNATE_ID and columnObjectTypeId as '0-1' as it's the email of the contact.
So the updated email column of the deal file will look like this -