File transfer format

Overview

If files are attached to a message, a special content type application/x-acro-filetransfer+json is used.

Format

{
    "body" : "Nice pics",
    "attachments" : [
        {
            "content-type" : "image/jpeg",
            "content-url" : "https://the.location.of/the/attachment.jpg",
            "content-size" : 20000,
            "filename" : "original_filename_if_any.jpg",
            "description" : "text for the particular attachment - not used so far",
            "encryption-key" : "0102f03cab.....",
            "hash" : "12638292",
            "preview" :
            {
                "content-type" : "image/jpeg",
                "content" : "BASE64"
            }
        },
        {
            "..." : " ... another attachment ... "
        }
    ]
}

body

Optional. Text applying to attachment(s)

attachments

The array of individual attachment dictionaries.

Field	Required	Description
content-type	No	If not present, image/jpeg is assumed.
content-url	Yes	Location from where to download the data.
content-size	No	Used to decide whether to download the file automatically or let the user start the download explicitly.
filename	No	If it makes sense to preserve it, for example when sending a named document from the filesystem, set it to the original filename on the sending device. It does not make sense to set it for a photo sent directly from the camera.
encryption-key	No	Hex-encoded AES128 CTR key. AES192 and AES256 are also supported. If present, the binary data downloaded from content-url needs to be decrypted using this key. Example script for decryption is available here.
hash	No	CRC32 digest of the decrypted data, or of the downloaded binary data when encryption-key is not present. If present, it should be used to verify integrity. Example script for decryption and integrity check is available here.
preview	No	Very low quality representation of the data to be downloaded. The preview image is used in the GUI until the actual content is downloaded and a better quality local preview is generated.

Preview object:

Field	Required	Description
content-type	No	If not present, image/jpeg is assumed.
content	Yes	BASE64 representation of the preview image.