Sharing a file in Slack via API is now asynchronous

Slack has an API to upload a file named files.upload that has already been deprecated. You now need to call files.getUploadURLExternal to get an endpoint to upload a file, upload a file, then call files.completeUploadExternal to get metadata of the uploaded file.

Unfortunately, unlike files.upload, metadata returned from files.completeUploadExternal might not be updated. For example, even when you upload a file and share it to a specific channel by specifying channel_id, returned metadata might not contain information about it.

You need to call files.info repeatedly until you get it when you want to get updated metadata, such as ts of a post for the shared file.