This multipart method creates an asynchronous background job that adds
or updates contacts by importing a CSV file containing contact information.
Do not use a Content-Type header value with this method.
Importing a new contact automatically sets the contact's permission_to_send
property as implicit
and the opt_in_source
property as Account
.
Importing an existing contact only updates the contact properties you
include in the request.
The CSV file has a maximum of 40,000 lines including the header row
(39,999 contacts) and a maximum file size of 4 megabytes (MB). Lines
above the 40,000 line maximum are not processed. If the request body
exceeds 4 MB, only the contacts contained in the first 4 MB are imported
and the remaining data is dropped.