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What it checks

For images: mimeType ∈ { image/jpeg, image/png, image/webp, image/gif }. For videos: mimeType === video/mp4. The mime is sniffed from the resolved bytes, not trusted from the file extension.

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Why

The PDS (Personal Data Server) blob endpoint enforces the same allowlist. Sending HEIC, BMP, MOV, etc. produces a BlobUnsupportedMimeType error from Bluesky — opaque, async, hard to debug. We catch it locally.

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Failure response

{
  "error": {
    "code": "preflight_failed",
    "rule": "bluesky.media.mime_allowed",
    "platform": "bluesky",
    "message": "Image mime type 'image/heic' is not allowed on Bluesky.",
    "remediation": "Use one of: image/jpeg, image/png, image/webp, image/gif."
  }
}

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Remediation

Re-encode the asset. ImageMagick, sharp, ffmpeg — any of them. For video, ffmpeg -i input.mov -c:v libx264 -c:a aac output.mp4 produces a Bluesky-compatible mp4.