thumb property
Optional. 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 multipart/form-data under <file_attach_name>.
Implementation
HttpFile? thumb;