I need to download a file with extension .IES. I´ve tried with other browsers and I can download It without problems. But when I try to do it in Firefox, the file´s extension changes from .ies to .pdf wich is a problem because users have to change manually the extension to the correct one. Is there a way to configure Firefox to don´t change file extensions?

I also try to configure downloads but that file extension doesn´t even shown on the list.