You have it correct.
Just make sure that you're not overwriting an existing .htaccess file. Depending on how you're accessing it, it may be there, but hidden. Most ftp clients can be configured to show hidden files. If you have ssh access, the 'ls -la' command will show hidden files. If one already exists, just add the AddType line to it somewhere.
You may have to make sure the .htaccess file is saved with unix-style line termination.
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