您不需要这些行来定义第二个边界(或随后的对该第二个边界的任何引用):

You shouldn't need these lines that define a second boundary (or any of the subsequent references to that second boundary):

Content-Disposition: form-data
Content-Type: multipart/mixed; boundary=e8bc9555e9634110bba63547b2552460

请尝试将其删除(以及随后所有对边界 e8bc9555e9634110bba63547b2552460 的引用)，以便您的请求如下所示:

Try removing that (and all subsequent references to boundary e8bc9555e9634110bba63547b2552460), so that your request looks like this:

POST https://demo.docusign.net/restapi/v2/accounts/295724/envelopes HTTP/1.1
X-DocuSign-Authentication: {"Username":"email","Password":"password","IntegratorKey":"key"}
Content-Type: multipart/form-data; boundary=9a56da749dc04804819460f6499ab80b
Accept: application/json
Host: demo.docusign.net
Content-Length: 31476
Expect: 100-continue

--9a56da749dc04804819460f6499ab80b
Content-Type: application/json
Content-Disposition: form-data

JSON_REQUEST_BODY_HERE
--9a56da749dc04804819460f6499ab80b
Content-Type:application/pdf
Content-Disposition: file; filename="ABC.pdf"; documentid=1 

DOCUMENT_1_BYTES_HERE
--9a56da749dc04804819460f6499ab80b
Content-Type:application/pdf
Content-Disposition: file; filename="AB.pdf"; documentid=2 

DOCUMENT_2_BYTES_HERE
--9a56da749dc04804819460f6499ab80b--