Slack integration uses a deprecated API

[zbodi74]

Describe the bug

Slack integration uses a deprecated endpoint to upload files.

https://api.slack.com/changelog/2024-04-a-better-way-to-upload-files-is-here-to-stay

The original web API method for uploading files to Slack, files.upload, is being sunset on March 11, 2025. Beginning May 8, 2024, newly-created apps will be unable to use this API method.

Apps & integrations should migrate away from files.upload and instead leverage a combination of two APIs: files.getUploadURLExternal and files.completeUploadExternal. The use of these two methods is more reliable, especially when uploading large files.

To Reproduce

n/a

Expected behavior

Use the endpoints recommended instead of the deprecated one.

Logs

No response

Information about your Metabase installation

master

Severity

P1 - beginning May 8 it won't be possible to set up a new Slack integration

Additional context

metabase/src/metabase/integrations/slack.clj

Lines 321 to 342 in dd66a69

	(mu/defn upload-file!
	"Calls Slack API `files.upload` endpoint and returns the URL of the uploaded file."
	[file :- NonEmptyByteArray
	filename :- ms/NonBlankString
	channel-id :- ms/NonBlankString]
	{:pre [(slack-configured?)]}
	(let [request {:multipart [{:name "file", :content file}
	{:name "filename", :content filename}
	{:name "channels", :content channel-id}]}
	response (try
	(POST "files.upload" request)
	(catch Throwable e
	;; If file upload fails with a "not_in_channel" error, we join the channel and try again.
	;; This is expected to happen the first time a Slack subscription is sent.
	(if (= "not_in_channel" (:error-code (ex-data e)))
	(do (-> channel-id
	(maybe-lookup-id (slack-cached-channels-and-usernames))
	join-channel!)
	(POST "files.upload" request))
	(throw e))))]
	(u/prog1 (get-in response [:file :url_private])
	(log/debug "Uploaded image" <>))))

[paoliniluis]