Where apps hide your files
The file was definitely in a channel. Whether it is on your Mac is a different question.
Files you explicitly download go to Downloads (configurable: Preferences → Advanced → Download location).
Everything else — previews, cached images — lives in:
~/Library/Application Support/Slack/as an Electron cache, in no user-serviceable form. The important consequence: a file merely *viewed* in Slack is not meaningfully on your Mac. If you need it later, download it now or be able to find the message later.
Slack's own search (in:#channel from:@person during:March) is the tool for "sent months ago" — the file lives on Slack's servers until retention policy deletes it, which on free plans it does.
Downloads go to Downloads by default. Files shared in channels actually live in SharePoint/OneDrive — the reliable copy is there, reachable via the Files tab in the channel or office.com. The local cache under ~/Library/Application Support/Microsoft/Teams (classic) or the containers of the newer app holds nothing worth retrieving directly.
Both platforms delete server-side files by policy — free Slack aggressively, corporate Teams per admin rules. A file that only ever existed in a channel is a file with an expiry date.
Anything from chat that matters gets saved into your own folders immediately, named so you can find it. Downloads-sorted-by-date answers "the thing from this morning"; nothing on your Mac answers "the spec from last spring" if it was never downloaded.
~/Downloads by default; configurable under Preferences → Advanced.
Not reliably — they live in SharePoint/OneDrive. Download what you need locally.
Everywhere keeps track of every file on your Mac — including the folders Spotlight hides and drives you have unplugged. Free for a day, then $19 once.
Download for Mac