How to provide a download link for a dxf inside a chm

Question

I have HTML code that compiles into a chm, and occasionally I want to include a link to directly download a file... for example a small binary drawing file (extension .qid in my app) used as sample data for a tutorial in the chm. I have been doing this just fine for little drawing files by just providing a link like this...

<a href="relativepath/some.qid" title="whatever">some text</a>

But my current problem is I have a little sample dxf that is to be used in this tutorial and when I provide a download link like this...

<a href="relativepath/some.dxf" title="whatever">some text</a>

...then I get a link ok, but when I click on it, it puts the dxf contents inline as text, rather than poppping up a download Save As dialog for some file at a path like mk:@MSITStore:wherever.dxf

Now I looked at HTML attribute doco and found a 'download' attribute which is meant to force the link to download, but it made no difference. I used this syntax...

<a href="relativepath/some.dxf" title="whatever" download>some text</a>

...which generated a chm with a link but ignored the attribute 'download'.

How can I force the href link to lead to a download dialog for a dxf file?