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The upload fails

Too large. Deployment upload limits apply before any platform limit. For big video, use upload-from-url so the file never passes through the browser. Unsupported type. Platforms accept a narrow set of formats. MP4 (H.264) for video and JPEG or PNG for images are accepted everywhere.

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The upload worked but publishing was rejected

The platform applies its own rules on receipt, which are stricter than the upload’s:
  • Aspect ratio — several platforms reject anything outside a range.
  • Duration — both minimum and maximum, depending on the platform.
  • Codec — a .mp4 container can still hold a codec the platform refuses. Re-encode to H.264 with yuv420p.
yuv420p is the one worth remembering. Video encoded as yuv444p plays fine locally and is rejected or renders black on most platforms.

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upload-from-url cannot fetch my file

The server fetches it with no credentials, so the URL must be publicly reachable. A signed URL works; a private bucket path does not.