Slack is ending support for their files.upload API on March 11, 2025.
You will now need to use the files.getUploadURLExternal and files.completeUploadExternal APIs to continue using upload functionality. Slack's changelog provides more information on this change, as well as migration steps: https://api.slack-gov.com/changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay
Our team is aware of this change and is working to update our Slack integration as quickly as possible. In the meantime, you should be able to use a REST Query to utilize the new APIs. Query REST APIs | Retool Docs
We apologize for any inconvenience and will keep you posted.