Attach a picture or video to a post
A post without a picture is a wall of text in a feed, and on LinkedIn nothing fetches one for you: the Posts API refuses to read the page a link points at, so a bare URL in the body renders as plain text with no card. Munin fills that gap in two ways, and knowing which one is in play decides how much work you have to do.
A draft that links somewhere already has a picture. At publish time Munin reads the
linked page, takes the image it advertises to the world (og:image), and uploads that with
the post. A draft carrying nothing but body and linkUrl goes out illustrated. You do
not have to do anything for this, and there is nothing to attach.
Attach a file when you want a particular one — a chart that makes the point, a clip of the thing working, an image someone made for this post. That is what the rest of this describes.
Tools
social_set_post_draft_media— attach or clear the media on a draft nobody has decided on.social_create_post_draft/social_propose_post_set— take the samemediaUrl,mediaKindandmediaAltTextwhen the file is known as the draft is filed.social_list_platforms— the accepted types and size ceilings, undermedia.cms_request_asset_upload+cms_complete_asset_upload— put a local file somewhere it has a URL.cms_upload_asset_from_base64— the same thing for something small you already hold.cms_upload_asset_from_url— copy a file that is already on the public web into the CMS.
What mediaUrl has to be
A public https URL that Munin can fetch at publish time — not now. The draft stores the URL and nothing else; the bytes are read when somebody publishes, and streamed straight to the platform. Two consequences worth holding on to:
- A URL that expires — a signed link good for an hour, a preview that dies with a deploy — is a draft that fails to publish tomorrow, long after you filed it.
- Munin never keeps a copy. Once the post is out, the platform holds its own; the draft keeps only the URL, for the record and for the preview in the review queue.
If the fetch fails at publish time, the publish is refused with social_media_failed and
the draft stays pending, deliberately: a post written around a picture is not the same post
without it. See skill://social/publish-a-reviewed-post.
A file on somebody's machine
There is nothing to point mediaUrl at, so give the file a home in the CMS first. Which
route depends on size:
| Size | Route |
|---|---|
| Up to 100 KB | cms_upload_asset_from_base64 — inline, one call, right for something generated in this conversation |
| Up to 50 MB | cms_request_asset_upload → PUT or POST the bytes yourself → cms_complete_asset_upload |
| Over 50 MB | Neither. cms_asset_too_large. The file needs a different public home |
The presigned route needs a runtime that can issue a raw HTTP PUT. If yours cannot, you cannot do this step, and the honest answer to the operator is to ask them to upload the file through the dashboard and hand you the URL — not to quietly file the draft without a picture.
SVG is rejected by the CMS, and LinkedIn does not take it either.
Then attach the asset's public URL with social_set_post_draft_media. The review queue
renders it — an image inline, a video with a player — so the person approving the post sees
what will go out rather than a filename.
An asset a draft is waiting on cannot be deleted
A social draft is not a CMS entry, so it does not appear under the entries half of
cms_list_asset_usage. It appears under elsewhere, and it counts: while a draft is
pending and points at an asset, cms_delete_asset refuses with cms_conflict, naming the
draft. A cleanup pass over "unused" assets will not quietly break a post that is waiting for
review.
The hold ends when the draft does. Publish it, dismiss it, or let it fail, and the asset is free to delete — which is correct, because the platform copied the bytes when the post went out. Deleting it then changes nothing about the live post.
Two things this does not cover: an asset uploaded for a post that was never filed as a draft is genuinely unreferenced and will be deleted, and an asset stored outside the CMS is nobody's business but yours. Name assets for the post they belong to and both stay manageable.
Video
Mp4, up to what social_list_platforms reports. Munin uploads it in parts, so a large file
takes a while and the publish call is slow rather than failing — do not retry it because it
seems stuck. LinkedIn then processes the video after accepting it, which is why a freshly
published video post can take a minute to look right on the platform.
Alt text
mediaAltText is what a screen reader announces. Write it for an image that carries meaning
— a chart, a screenshot with words in it — and describe what the picture says, not that it
is a picture. An image that is pure decoration can go without. The platform stores it with
the post; Munin shows it under the preview in the review queue.