Thumbnail of the file sent; can be ignored if thumbnail generation for
the file is supported server-side. The thumbnail should be in JPEG
format and less than 200 kB in size. A thumbnail's width and height
should not exceed 320. Ignored if the file is not uploaded using
multipart
form-data. Thumbnails can't be reused and can be only
uploaded as a new file, so you can pass
“attach:file_attach_name” if the thumbnail was uploaded
using multipartform-data under
file_attach_name. More info
on Sending Files »