Add argument for attachment thumbnail size generation

[bnvk]

Currently there is no way to specify generating different size thumbnails. There is only the /message/download/preview/={{ mid }} request which returns the one size thumbnail.

One approach (perhaps the most versatile to designers) is allow a simple ?height=200&width=300 query string arguments.

[ritvikagrawal1]

i'd like to try and pick it up @bnvk