files.uploadV2 image alt text is not saved by Slack #2788

Package

@slack/web-api

SDK Version

@slack/web-api@8.2.0. Also seen on 7.19.0. The current main (8730b5e) has the same code.

Node.js Version

v24.21.0

Operating System

macOS 27.0.1

Steps to Reproduce

1. Create a bot with the files:write and files:read scopes, and invite it to a channel.

2. Upload an image with alt text:

   await client.files.uploadV2({
     channel_id: 'C0123456789',
     file: './image.png',
     filename: 'image.png',
     alt_text: 'A blue cat',
   });

3. Wait a few seconds. Then call files.info for the uploaded file and check alt_txt.

Expected Result

files.info returns alt_txt: 'A blue cat', and the image shows an ALT badge on hover in the Slack client.

Actual Result

files.info has no alt_txt field, and the image shows no ALT badge on hover.

Cause: uploadV2 passes the text to files.getUploadURLExternal as alt_text. The API expects alt_txt (docs), so Slack ignores the value. The request body looks like this:

filename=image.png&length=…&alt_text=A%20blue%20cat

When I changed only this key to alt_txt in a local build, files.info returned the text and the ALT badge appeared.

The request type FilesGetUploadURLExternalArguments declares alt_text but not alt_txt, so it does not match the API either.

Fix: #2789