Slack has never published a list of video formats that play inline in a message. Not for WebM, not for MP4, not for anything else. That absence is the whole story here: "WebM is supported" is something people inferred, not something Slack said.
Your WebM almost certainly uploaded fine. What failed is playback — a different system with a different set of rules, and the one Slack has not documented.
Upload and playback are separate — work out which one broke
Slack is explicit about the upload side. From the "Add files to Slack" help article: "You can add files up to 1GB in size." That sentence appears without qualification — there is no per-plan table on the page splitting the number across Free, Pro, Business+ and Enterprise+. So if your .webm is an ordinary screen recording, it went up: you can see it in the channel with a filename and a size.
Playback is a different question, and Slack's documentation does not answer it. The only preview rules Slack publishes are these, verbatim from the same article:
"Slack will include a preview of a file as part of your message. If you share an image, the preview will only display inline if it's smaller than 25,000 pixels on the longest side, and less than 45 million pixels total. Microsoft Office files that are larger than 50 MB will not display a preview."
That covers images and Microsoft Office files — the entire published rule set. There is no equivalent sentence for video — no resolution ceiling, no bitrate rule, no container list, no codec requirement. If you are hunting for the threshold your WebM crossed, there is no documented threshold to cross.
One note, since these get misquoted: the two image conditions are joined by and, not or, and they govern display, not upload. A huge PNG still uploads; Slack just declines to render it inline.
Where "Slack supports WebM" comes from
Slack's developer documentation publishes a file object reference containing a table of filetype values Slack recognises. Among the audio and video entries:
flv Flash video
mkv Matroska video
mov QuickTime video
mp4 MPEG 4 video
mpg MPEG video
ogv Ogg video
webm WebM
wmv Windows Media Videowebm is right there with a proper name beside it, so the conclusion writes itself: WebM is supported.
It does not follow. That table enumerates the labels Slack's API can attach to a file. It is a type taxonomy, not a playback whitelist. Nothing on that page promises that any of those containers renders in a message — it describes how files get classified, not what the client does with them.
The giveaway sits in the same table. wmv has an entry. So does flv. Nobody seriously believes Slack plays Flash video inline in 2026, yet it appears there for exactly the same reason: Slack needs a label for the file, so Slack has a label for the file. A row tells you what Slack will call your upload, not whether it will play. Most third-party "Slack supported video formats" articles are this table reprinted with a promise the source never made.
The one official sentence that names video codecs — and why it does not apply
VP9 and AV1 do appear in Slack's documentation, once — in the guide to network and system configuration for huddles:
"Huddles rely on AV1, VP9, and H.264 for media encoding and decoding. To ensure these codecs are optimally supported, update your users' devices and graphics drivers regularly."
Borrowing that sentence is a mistake. It is scoped to huddles — Slack's real-time calls. It sits in a document about network configuration and graphics drivers, describing what the live media pipeline encodes and decodes between participants' devices. It says nothing about a file you drag into a message field.
A codec negotiated live between clients in a call and a codec decoded from a stored file are different parts of the product, and Slack has documented only the first.
The fix: convert to MP4 before you send it
Since there is no documented answer to appeal to, change the variable you control: re-encode the WebM into an MP4 and post that instead.
Try it right here
Runs in your browser. Your file is not uploaded anywhere.
This runs inside your browser tab. The file is read from disk, converted locally, and handed back — nothing is uploaded to anyone's server, which matters when the clip is an internal recording meant only for your team. WebM to MP4 is the same converter on its own page. If the file in question is a .mov instead, Slack MOV file not playing covers that case.
- Check the size afterwards. WebM and MP4 do not compress identically, and a re-encode can land larger than the file you started with. With 1GB of headroom this rarely bites, but if you are near it, run the result through video compression too — how to compress a video without losing quality covers which settings move the number.
- Keep the MP4 as the copy you share. If you also need WebM for a web page, make that separately with MP4 to WebM.
Other causes, in order of likelihood
Work down this list if converting did not settle it.
1. Free workspace, and the file has aged out. Symptom: the message is still there, the file is not, and the thread is months old. Slack: "When your workspace reaches the visibility limit, Slack will start hiding messages and files older than 90 days to make room for new ones. Messages and files more than one year old will be permanently deleted." Two different mechanisms. Ninety days is hiding — "the remaining 275 days will become available should you upgrade to a paid plan." One year is deletion: "It will not be possible to restore messages and files once deletion occurs."
The trap: Workspace Owners choose between basic retention settings — "either keep all messages and files for one year, or delete them after 90 days." If your owner picked the delete option, content hidden by the 90-day limit is deleted automatically, and upgrading will not recover it.
2. The upload never finished and you are looking at a stub. Slack scans everything: "Files you add to Slack will be scanned for viruses and other malware, and won't be uploaded if any are detected." And 1GB may not be your ceiling — Slack's developer docs define file_upload_size_restricted as "The size of provided file is too large, as the team has restricted uploads of large files," so a workspace can be stricter than the published number. Free teams can also hit storage_limit_reached: "File storage limit has been reached. This occurs when free teams have uploaded 5GB of files." That 5GB is a workspace-wide storage total, not a per-file cap, and it comes from a developer error-code table plus a 2019 changelog, not a current user-facing help page.
3. You are on mobile and trying to fall back to downloading. Slack: "If you'd like to save files added to Slack to your desktop, you can download them. On your mobile device, you can only download image files." So "download it and open it in a proper player" is desktop-only. On a phone, a video that will not play in Slack leaves the reader with no official escape hatch.
4. You zipped it to be helpful. In Slack Connect channels and DMs — the scope matters, this is not workspace-wide — Slack blocks a list of extensions: "To help keep Slack secure, certain types of files cannot be added to Slack Connect channels and direct messages (DMs)." .7z, .gz and .tar are on that list. .mp4, .mov, .mkv and .webm are not. Archiving a video for an external partner turns a file that would have gone through into one that gets refused.
5. You are thinking of clips, which are a separate feature. Slack caps them: "Share audio and video clips up to five minutes long in Slack to communicate easily with your team ..." Five minutes belongs to clips; Slack publishes no duration limit for ordinary video uploads. Clips do let you "Upload an existing video as a clip," but Slack does not state which containers or codecs that path accepts — check the official documentation if you need that.
How to tell it worked
Post the converted MP4 into a DM with yourself first. Play it in the Slack client you actually use, then in the mobile app if your audience is on phones. If it plays on desktop only, tell people to open it on a computer — on mobile they cannot download a video to work around it. Keep the original WebM until that check passes.
FAQ
Does Slack support WebM?
Slack's API recognises webm as a filetype value, so Slack will label your file correctly — which is not the same as playing it. Slack publishes no list of video formats that play inline in a message, for WebM or anything else.
Why does my MP4 play in Slack but my WebM doesn't? Slack does not document a reason, because it does not document inline video playback at all. There is no published container or codec requirement to check against. Converting to MP4 is what changes the outcome.
What video formats does Slack support?
Slack has not published a supported-formats list for video playback. Lists elsewhere are usually reprints of the API's filetype table, a classification taxonomy that includes flv and wmv too. For anything authoritative, check the official documentation.
What is the maximum video size I can upload to Slack? Slack's help center states "You can add files up to 1GB in size," with no per-plan breakdown. Your workspace may be configured lower, though — Slack's developer docs describe an error for teams that have "restricted uploads of large files."
Can I share a link to the file instead of posting it? Only on a paid workspace: "If your workspace is on a paid plan, you can create links to share files outside of Slack." Slack adds that in a private channel, only the person who uploaded a file can create an external link for it.
Is it safe to convert a work recording online? It depends on whether the tool uploads your file. Most "online video converter" sites send it to their servers. The converter on this page runs in your browser and never uploads anything. See all Clapr tools for the rest.
Quotes from Slack's help center and developer documentation, checked 16 September 2026. Verify against the official source before relying on them.
