Set Content-Disposition on MIME body part on Content-Type multipart/form-data

I’m following this thread

I’m able to generate the Mime Message correctly but the Content-Type generated by IS is multipart/mixed instead of multipart/form-data, are they both the same?

User-Agent: Mozilla/4.0 [en] (WinNT; I)
Host: localhost:5555
Authorization: Basic QWRtaW5pc3RyYXRvcjptYW5hZ2U=
Content-Type: multipart/mixed; boundary="----=_Part_2_272149066.1659948795948"
Date: Mon, 8 Aug 2022 15:53:15 +0700 (ICT)
Message-ID: <609641145.3.1659948795953@VincentKo>
MIME-Version: 1.0
Content-Length: 718

------=_Part_2_272149066.1659948795948
content-type: text/plain
content-transfer-encoding: 7bit
Content-Disposition: form-data; name=jsonListObjectGadai

{"channel":"MSM","deviceId":"VAMS"}
------=_Part_2_272149066.1659948795948
content-type: text/plain
content-transfer-encoding: 7bit
Content-Disposition: form-data; name=partnerReferenceNumber

123123123
------=_Part_2_272149066.1659948795948
content-type: text/plain
content-transfer-encoding: 7bit
Content-Disposition: form-data; name=gadaiVar

[{  "BeratEmas": 10000,  "JumlahEmas": 1,  "Karatase": 17,  "Karatase_title": "17 Karat",  "ObjectGadai": "Logam Mulia/Batangan",  "ObjectGadaiCode": "LM1"}]
------=_Part_2_272149066.1659948795948--