Return `dataset` name in form attachments API in Backend

There are a couple places in Frontend (and Backend) where a dataset is deduced from an attachment by removing a `.csv` extension from the attachment. The code is pretty straightforward but there could be some capitalization inconsistencies. This has been [centralized in Backend](https://github.com/getodk/central-backend/blob/master/lib/util/util.js#L83) and we could let Backend return the actual Dataset name for an attachment when it knows it is a dataset.