Actually, I just realized that if you wanted to open the file with in the browser itself (as opposed to download prompt), you could edit the web.xml file (back it up first) and remove the extension you do not want the browser to do a Save As.
Save and restart SDM Tomcat
pdm_tomcat_nxd -c stop
wait a minute
pdm_tomcat_nxd -c start
Retest now.
Unless your browser is forcing that file type for a download (browser policies controlled by network admins), SDM should not inherently force you to download the file. The file is still downloaded by the browser to its Temp internet files folder as part of its normal operation.
_R