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Find your symptom and open it. If you are hitting a size limit, read the first box before anything else: the number that catches people is not the one they expect.
POST /public/v1/posts accepts about 100 kB, not 50 MB. The 50 MB body limit is mounted on /posts and /copilot/*, which are the internal routes the web interface uses. The public API path is neither, so it falls back to the framework default, and a single inlined image clears it.Upload the file first and reference the returned path.
You inlined image data instead of uploading first. Upload through /public/v1/upload or /public/v1/upload-from-url, then pass the returned id and path in the post body’s image array.
/public/v1/upload checks the file’s detected MIME type against this list:/public/v1/upload-from-url applies the same list to the bytes it downloads, but checks the URL first: the path, ignoring any query string, must end in .png, .jpg, .jpeg, .gif, .webp or .mp4.So AVIF, BMP and TIFF cannot be fetched from a URL at all, only sent through the multipart endpoint. A 400 naming extensions rather than MIME types is how you tell the two checks apart.PDFs are not accepted. The only PDF flow is LinkedIn document carousels, produced internally by converting an image carousel.
Per-file caps, on both endpoints:
The backend proxies the source URL, so it fails when the source is slow, unreachable, blocks the user agent, or sits behind authentication.
  • The source must be publicly reachable HTTPS with no auth
  • Not private S3 URLs, expired signed URLs, or intranet hosts
  • Consistently slow source? Download it and use the multipart /upload endpoint
She inspects duration, dimensions and codec before handing the file to the network, and files outside the supported range fail here.Other formats may upload and then be rejected by the network downstream.
You pressed Cancel on a file already in flight, and the uploader does not support that. Wait for it to finish, then delete the asset from the post.A known limitation, tracked in Known Issues.

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