Adding the "type" param to hrefs is only a browser fallback option and won't override any server prefs that may already be defined.
Any of the AddType or AddHandler statements may be added to the .htaccess file (in the folder where the file is going to be served from--or the web root folder) to override server settings in the case where you don't have full control of your server configuration. Something like:
AddType application/x-mobipocket-ebook .mobi
should do the trick.
NOTE: this is all assuming your web server is Apache and your provider is not preventing Overrides. Don't mess if you don't know, as improperly written .htaccess files can render your site unusable.
EDIT: render it unusable until you fix or delete the bad .htaccess file--didn't mean to make it sound as drastic as I did.