curl --request POST \
  --url https://sandbox.api.fin.com/v1/customers/upload \
  --header 'Authorization: Bearer <token>' \
  --header 'Content-Type: multipart/form-data' \
  --form customer_id=659fd1e8-b7db-4276-8c0f-33b354fe68ef \
  --form file1='@example-file' \
  --form file_foo='@example-file' \
  --form file_bar='@example-file' \
  --form [what_ever_you_want]='@example-file'

{
  "data": {
    "files": [
      {
        "file1": "/AbAcQ4hn_0652746727637.pdf"
      }
    ]
  }
}