`files_upload_v2` Doesn't work with user ID's or channel names

What was going on
Issue slackapi/python-slack-sdk#1326 (closed, 13 comments): With the older function `files_upload` you used to be able to use channel names such as "#my_channel" for the `channels` argument. Or use user ID's as the `channels` argument to send the file to a user as a direct message. The new `files_upload_v2` doesn't work with either channel names or user ID's. And seems to only accept channel ID's for the `channel` argument now. So unless I'm missing something there's currently no way to use `files_upload_v2` to send a direct message to a user? it must be posted to a channel.

The fix
files_upload_v2 only accepts channel IDs, not channel names or user IDs: confirmed as intended by the Slack team. Channel name support was deliberately omitted from the v2 upload endpoints (completeUploadExternal) because names can be renamed while IDs stay stable; apps keyed on names break more easily. Pass channel IDs; if you only have a name, resolve it to the channel ID first (e.g. via conversations_list) before uploading.