Export keys from dataset does not render json output

Steps to reproduce

1.) Create a pool with encyrption or not (this does not matter)

2.) Crete an encrypted dataset with keys different from pool (if pool is encryped then uncheck inherit)

3.) Click gear next to pools -> Export dataset keys and save the file

4.) Expand pool to show datasets, click hamburger menu next to dataset, and click export key

Result

1.) Key exported from pool outputs to json format (pass)

2.) Key exported from dataset outputs to txt format (fail)

The impact is that the text file has to be converted to json format manually if export by dataset level is used.