Save a file with the extension I want
Apparently this is a chrome bug that's been around for some time and never seems to have been fixed. 

Several bug reports are available but I haven't ever seen a fix or even a chrome extension that fixes it.  I think if the web server identifies the content or mime type correctly it might work, but looks like there's nothing you can do about that from the browser-side of things. 

http://code.google.com/p/chromium/issues/detail?id=38213
http://code.google.com/p/chromium/issues/detail?id=39708&amp;q=download%20filename%20extensions&amp;colspec=ID%20Pri%20M%20Iteration%20ReleaseBlock%20Cr%20Status%20Owner%20Summary%20OS%20Modified