I'm not sure how much advantages a preview option for downloading would have. After having done the regular download, one sees the not yet started image (with the ⏳ next to it), then you can inspect and delete it, if you want. Or is there some more things you could do with preview?
Thanks for the feedback. When you do the + and select branch, a rebase button appears, When you press that, it will rebase. There is currently no fetch option.