Any file that is issued by HTTP protocol has an additional header, which indicates its type. He is given the title Content-type. If it is clearly wrong - for example, shaking 7z archive, and in the type of written text / html, then it is a problem with the server configuration, "Content-Disposition" header field, I guess.
No opportunity to edit Preferences-File Handling-By type. I tried to change one of the records (eg 3gpp) manually in the browser files (and then edit an existing entry, giving her property "Save them on my computer"), but the attempt was unsuccessful. Maybe someone did it?
I apologize for the translation with Google