PNG transparency in thumbnails

Hi

im using the Javascript API to grab a few thumbnails for images in my app folder

however wehn i grab the thumbnail for a png with transparency / alpha the transparent part shows as white in the thumbnail

this is the method i am using to get the thumbs

dbx.filesGetThumbnail({path:path,size:{['.tag']:'w640h480'},format:{['.tag']:'png'}})

the raw images do have transparency as i have check by replacing the line above with

dbx.filesDownload({path:path})

and i get all images corently with their transparency.

does the API method filesGetThumbnail support thumbs with transparency ?

or do i have a mistake in the above?

Thanks

Adam