Using Create Guild Sticker endpoint with `payload_json` results in inconsistent behavior

[TTtie]

Description

Using payload_json with the Create Guild Sticker endpoint (seemingly implemented in #3969) results in unexpected and inconsistent behavior contrary to other endpoints using payload_json in the API.

Steps to Reproduce

1. Send the following request

   curl --trace-ascii - -s -H"Authorization: Bot $BOT_TOKEN" \
       -F"file=@file.png" \
       -F'payload_json={"name": "testing","description": "testing sticker","tags": "tag"};type=application/json' \
       https://discord.com/api/v10/guilds/$GUILD_ID/stickers

2. Send the following request

   curl --trace-ascii - -s -H"Authorization: Bot $BOT_TOKEN" \
       -F"name=test" \
       -F"description=desc" \
       -F"tags=tags" \
       -F"file=@file.png" \
       -F'payload_json={};type=application/json' \
       https://discord.com/api/v10/guilds/$GUILD_ID/stickers

3. Send the following request

   curl --trace-ascii - -s -H"Authorization: Bot $BOT_TOKEN" \
       -F"name=test" \
       -F"description=desc" \
       -F"tags=tags" \
       -F"file=@file.png" \
       -F'payload_json={"name": "testing","description": "testing sticker","tags": "tag"};type=application/json' \
       https://discord.com/api/v10/guilds/$GUILD_ID/stickers

Expected Behavior

• Not providing name, description, and tags and supplying them through payload_json (request 1) works as expected and creates a sticker.
• name, description, and tags multipart fields are ignored when payload_json is supplied and values from payload_json are used instead, as is with the Create Message and Execute Webhook endpoints (not the case when executing request 3)

Current Behavior

• Not providing name, description, and tags multipart fields and supplying them through payload_json results in HTTP 400 Bad Request (as seen in request 1)
• Providing an empty object in payload_json along with supplying multipart fields results in error 50035 (Invalid Form Body) (as seen in request 2)
• Providing name, description, and tags in both multipart body and payload_json results in the values from the multipart body being used (as seen in request 3)

Screenshots/Videos

No response

Client and System Information

Discord API v9/v10, any client (tested with curl 7.82.0)

[DV8FromTheWorld]

Confirmed that there is a bug here.

• Request 1: This should just work and I've found the bug. Will be opening a PR
• Request 2: This should fail with a 400 indicating that name and tags are required. When you provide payload_json it becomes the source of truth for the request, so the parameters not being in there causes the 400.
• Request 3: This should just work and I've found the bug. Going forward, like with Request 2, payload_json will once again be the source of truth.

Thank you for the test cases. I'll update this once the fix is merged and deployed!

[DV8FromTheWorld]

A fix for this has been merged and will be available in the API following deployments.

[DV8FromTheWorld]

This is deployed