-  download_button(+Data:string, +Options)
 - Emit a button in the SWISH output window for downloading Data.
The provided data is stored on the server.
Options:
- filename(+Name)
 - (Base-)Name of the file created (default:
swish-download.dat), 
- content_type(+Type)
 - Full content type. By default this is derived from the
extension of the filename and the encoding.
 
- encoding(+Enc)
 - Encoding to use. One of 
utf8 or octet. default is
utf8. 
- See also
 - - https://en.wikipedia.org/wiki/Data_URI_scheme