There's no icon available, but I'd like to suggest a different course of action: we offer a "Download image/video/..." button inside the read-only editing form opened by the "Show versions" action instead. This allows me to preview a particular version and directly download it.
The button would be a main action (i.e. a gray button in the bottom bar), but not the default button. We should probably make the "Cancel" button the (green) default button at the same time. Since the download action is not directly related to "Cancel", I'd move it the left side of the bar, similar to how we do it in the chooser dialog.
A download button only needs to be available/shown, if the asset's media type supports a download. It should be shown but disabled, if the asset does not actually contain a medium.
This has the following advantages:
- you can preview, before you download
- you can open the preview of a version from anywhere else, and you'd still be able to download the medium.
- the actions inside the "versioning" group focus on handling versions (previewing/comparing/restoring them), similar to their counterparts in Pages.
- we don't have to add another action to the standard Action bar in Assets.
Ideally, we would have a "download" action on the media upload field itself, especially since we might have more than one medium inside an asset (or similar content type). Actually, this will have to be changed accordingly. Similarly, we would offer the same "download" button also inside the regular "add/edit asset" dialog. There, again, the button would actually have to reside on the media upload field itself. I'll add follow-up tickets for that.