Inconsistent downloadable behaviour

[fsuter]

The preview features allows for downloading the previewed data. But there's a confusion in the underlying code:

• first of all, there are 2 ways to indicate that the data is downloadable. The consistency of this mechanism has to be reviewed. Ideally there should be a single operation needed to indicated downloadability.
• the downloadable data is that stored in the records variable of the Data object, which may not be the same as the preview data. Again, consistency must be ensured.