Thanks for your reply.

The first response highlighted the filename - upon further research, I see that the MIME type header can be enhanced with...
Content-Disposition: attachment; filename=<name>.slk;
... and this now works without giving annoying messages. (The browser layer is < 1% of our code base and we don't know the technology.) The key part is the filename extension of SLK: the MIME header provides a filename rather than the browser creating a name on the fly. This gives the control over filename/contents that we require.

Apologies for the missing images, but I assumed that since they paste in successfully then they are accepted for later display.