Upload a supporting document for a customer: a KYB document, a
control-person identity document, or payment documentation. See
FileType for the full list. Payment documents are single-use;
upload a new document per payout.

file_type and control_person_id are multipart form fields (a
file_type query parameter is still accepted for backward compatibility).
On a BUSINESS customer, control_person_id is required when file_type
is a person-level document (IDENTITY_DOCUMENT, IDENTITY_DOCUMENT_BACK or
PROOF_OF_ADDRESS) so the document is attached to the specific control
person it belongs to, and must be omitted for all other file types. It must
reference a control person that exists on this customer. On an INDIVIDUAL
customer, documents belong to the customer and control_person_id must be
omitted. SELFIE and SOURCE_OF_FUNDS are accepted only for
INDIVIDUAL customers.