Messenger error 100: Upload attachment failure

Code 100 with subcode 2018047 is the attachment-processing failure, and Meta's error-codes table names the most common trigger in the message itself: "the provided media type does not match type of file provided in the URL." Send API attachments declare a type, image, audio, video or file, and the platform verifies the declaration against what it actually downloads. Declare image and serve an MP4, or declare video and serve a URL whose response is HTML, and the upload is rejected.

Mismatches sneak in through infrastructure more often than through code. URLs without file extensions that your code classifies by guesswork; CDNs that respond with a generic application/octet-stream; image proxies that transcode JPEG requests to WebP; signed URLs that expire between classification and Meta's fetch, turning the response into an XML error document. The declared type is also how the message renders, an audio file sent as file loses its player, so the type must be right, not merely accepted.

Unsupported or exotic formats fail here too: formats outside Messenger's renderable set are best sent as file, and images with unusual encodings (CMYK JPEGs, animated WebP) are safer transcoded to the mainstream (JPEG, PNG, GIF, MP4) before sending. Its neighbors bound the rest of the media story: 2018008 when the fetch fails outright, 2018109 when the file is too big, 2018294 when video fetching times out.

{
  "error": {
    "message": "(#100) Upload attachment failure.",
    "type": "OAuthException",
    "code": 100,
    "error_subcode": 2018047,
    "fbtrace_id": "BLBz/WZt8dN"
  }
}

Derive attachment.type from the file's verified MIME type, not its extension or the caller's claim, one shared classifier used by every send path. Standardize bot media on the mainstream formats at upload time, and prefer reusable attachment_ids for anything sent more than once. The supported types and size caps are tabulated in the Messenger limits reference, and media troubleshooting has a section in the Messenger guide.