Name: jmap_imagesize_annot
Type: STRING
Default-Value: NULL
Last-Modified: 3.1.1

The name of the per-message annotation, if any, that stores a JSON object,
mapping message part numbers of MIME image types to an array of their image
dimensions. The array must have at least two entries, where the first entry
denotes the width and the second entry the height of the image. Any
additional values are ignored.

For example, if message part 1.2 contains an image of width 300 and height
200, then the value of this annotation would be:

{ "1.2" : [ 300, 200 ] }
