Files saving as .mpp by default

Hi,

I am sending a new mpx file to the client browser by writing it to Response.OutputStream, but the IE save dialog uses .mpp as the extension if none is specified. MS then can't read the file because it should be .mpx.

It appears that setting ContentType to "application/" (or any other invalid type) instead of "application/vnd.ms-project" will cause the file to save with the correct extension by default, but using "application/octet-stream", IE somehow figures out that it's a project file and still tries to save it as an mpp.

Is there an acceptable way to specify that the file should be saved with .mpx instead of .mpp?

Thanks,
Matt