Can't reproduce. Some special Tab Mix Plus settings (or other extensions)? And what you mean? Click on some "downloadable" link -> Opening ... dialog -> check Save File radiobutton -> Ok or you use "Save link as… (Ctrl+S)" button on Link properties dialog?
Anyway please provide detailed description here: https://github.com/Infocatcher/Link_Properties_Plus/issues Information from about:support page and exported Tab Mix Plus settings may help me to fix this.