Whether WordPress accepts a .webm upload depends on which WordPress you are running — and the two sets of official documentation say different things in writing.
This is not a rumour or a version quirk. It is two current, official pages giving opposite impressions, and once you work out which one governs your site, the rejection stops being mysterious.
What this error actually means
WordPress gates uploads on file type, not only on file size. WordPress.com states this plainly on its Accepted file types page: "WordPress.com restricts uploads to the file types listed on this page. Restricting file types protects your site from unsafe files."
The wording you see when a type is refused is documented, and it differs depending on where your site lives:
- Self-hosted, from the Audio block documentation: "Sorry, this file type is not supported here."
- Multisite networks, from the Network Admin Settings screen documentation: "Sorry, this file type is not permitted for security reasons"
- WordPress.com, from its upload troubleshooting page: ""This file cannot be processed by the web server." The file type is not supported. This message is not about the file's size."
That last sentence is the one to hold onto. WordPress.com says outright that this message is not about the file's size, which rules out the first thing most people try — compressing the video and uploading it again.
The two official answers on .webm
What WordPress core says
developer.wordpress.org publishes the body of wp_get_mime_types(), the core function that defines the default extension-to-MIME-type map. Its Video formats section contains, verbatim:
'webm' => 'video/webm',
'mkv' => 'video/x-matroska',So in the open-source software distributed by the WordPress project, webm is in the default map — sitting next to mkv, mp4|m4v, mov|qt and the rest.
One limit on what that proves: the core reference documents this as a map from extensions to MIME types. It is not a codec whitelist, and WordPress's official documentation doesn't state which video codecs are supported. The map only decides whether the file is allowed through the door.
What WordPress.com says
The WordPress.com help centre's Accepted file types page lists, under Video: .mp4, .m4v, .mpg, .mov, .vtt, .avi, .ogv, .wmv, .3gp and .3g2. There is no .webm on that list — and the same page names it by hand in its Other formats section:
"If you need to upload a file extension not listed on this page, such as .mobile, .epub, or .webm, you can add support for these file types via a plugin—plugin-enabled sites run on more advanced infrastructure that can support file types not normally accepted."
Before you assume VideoPress is the way around it, note that VideoPress publishes its own, shorter list — and WebM is not on it either:
"VideoPress supports a wide range of video formats, including: MP4 (.mp4, .m4v) QuickTime (.mov) Windows Media Video (.wmv) AVI (.avi) MPEG (.mpg) Ogg (.ogv) 3GPP (.3gp) 3GPP2 (.3g2)"
That list is introduced with "including", so it is not presented as a complete whitelist — but nothing official adds .webm to it.
One more scope line worth reading on the Accepted file types page, at the head of its Video section: "This section of the guide applies to sites with our WordPress.com Premium, Business, and Commerce plans. For free sites and sites on the Personal plan, upgrade your plan to access this feature." On a free or Personal site, the file extension may not be the thing that stopped you.
Two official URLs, side by side:
- Core function reference: https://developer.wordpress.org/reference/functions/wp_get_mime_types/
- WordPress.com Accepted file types: https://wordpress.com/support/accepted-filetypes/
Neither page is wrong. They describe two different products, published by two different organisations, and they are the reason the same file behaves differently on two WordPress sites. Check which of those two pages describes your site before you spend an hour on plugins.
The fastest fix: convert the file to MP4
WordPress.com's own troubleshooting page prescribes exactly this: "If you have images and other files in unsupported formats that you want to use, then you should convert them to a valid file type before uploading. You can do this with editing software or a free online converter."
MP4 is the target with the most official backing on both sides. .mp4 and .m4v are on the WordPress.com Video list, on the VideoPress list, and in core's map as 'mp4|m4v' => 'video/mp4'.
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, processed locally, and handed back. It never touches a server, which matters when the video is a client draft or an unpublished course lesson.
If your problem is the reverse — you are on a self-hosted site and deliberately want WebM for delivery — MP4 to WebM is the same tool pointed the other way.
Other causes, in order of likelihood
1. You are on a Multisite network. A network has its own Upload file types field, separate from core's map, and the documentation is emphatic about how it works: "Adding mime types in the 'upload file types' field not listed in the default set will NOT work unless you've added them using the upload_mimes filter! Uploading files with mime types not supported (without adding them using the filter) will fail with the message "Sorry, this file type is not permitted for security reasons"." If that is the exact wording you are seeing, look at the network settings, not at your site.
2. A plugin or theme changed the allowed list. Core's wp_get_mime_types() reference notes, of its own filter: "This filter should be used to add, not remove, mime types. To remove mime types, use the 'upload_mimes' filter." So upload_mimes is the documented way to take a type away — meaning any plugin on your site can remove webm from a list that core shipped with it.
3. Your user role. The types actually offered are produced by get_allowed_mime_types(), not by wp_get_mime_types() directly. Its body removes swf and exe unconditionally, and removes htm|html and js only when the user lacks the unfiltered_html capability. The allowed set is therefore per user, not one table for the whole site. Note what that code does not touch: webm is not among the types core strips out, so a self-hosted rejection is not core's default behaviour.
4. It is not a type problem at all. On a self-hosted install the maximum upload size comes from PHP, not from WordPress: wp_max_upload_size() is documented as determining "the maximum upload size allowed in php.ini", and returns the smaller of upload_max_filesize and post_max_size. A size failure produces a different message from the type failures quoted above — match the wording before you start editing configuration.
How to tell it worked
Upload the converted MP4 and check the Media Library listing rather than trusting the progress bar. If the item appears, the type gate passed. If you get a different message than before — particularly one about size or the server — you have moved on to a separate problem, and the type question is settled.
For more upload and export failures in this format, see the fix guides hub, or browse all Clapr tools if you need a different conversion.
FAQ
Does WordPress support WebM or not?
Both answers are official. WordPress core's wp_get_mime_types() includes 'webm' => 'video/webm' in its default map. WordPress.com's Accepted file types page does not list .webm and names it as an extension you would add via a plugin. Which applies depends on whether your site is self-hosted or on WordPress.com.
Why does the same .webm upload on one WordPress site and fail on another? Because the two sites are running under different rules — core's default MIME map versus the WordPress.com accepted list — and because plugins, Multisite network settings and user capability can each change the list on a given site.
Is "this file type is not supported" caused by my file being too big? No. WordPress.com's troubleshooting page says of that message: "This message is not about the file's size." Converting the file to an accepted type is the fix; compressing it is not.
What should I convert WebM to?
MP4. It is on the WordPress.com video list and in core's default map as 'mp4|m4v' => 'video/mp4'. You can do it in the browser with WebM to MP4.
Can I just add WebM support with a plugin?
On WordPress.com that is what the Accepted file types page itself points to for extensions not on its list. On Multisite, the network documentation warns that adding a type to the Upload file types field will not work on its own unless a function is also hooked to the upload_mimes filter.
Quotes taken from WordPress's official documentation at developer.wordpress.org and wordpress.com/support, as recorded on 16 September 2026. Both sets of pages are revised regularly — check the official documentation before relying on them.
