Slack's new upload mechanism

[oizyumov]

Describe the bug
Slack informed its users that files.upload API is going to be suspended:

May 8, 2024
Newly created Slack apps will no longer be able to access the files.upload API. Existing applications will be able to continue using files.upload until support is discontinued.

March 11, 2025
Support for the files.upload API for all apps will be discontinued.

To prepare for this change, we recommend that app owners migrate away from files.upload and instead use the combination of files.getUploadURLExternal and files.completeUploadExternal. You can also leverage Slack's SDKs to help transition to this new way of uploading files by visiting our api.slack page.

If the apps using the files.upload API do not update to the method above, file uploads from those apps in your workspace or organization will no longer work beginning on March 11, 2025.

To Reproduce
So the new users won't be able to bridge icons from other platforms into slack, while bridging messages.
And existing users will probably get an error in a year.