1. Log into the Customer Control Panel with your main account's username and password.
2. Once logged in you'll see a link that says "Login via IP Address" under the Parallels Plesk Panel section at the bottom left hand side of the Home page.
3. Once you click on that link, a new window will come up, you might get a certificate warning, just click continue, then your Plesk Panel will come up.
4. Click on "Domains" on the left hand column.
5. You'll see the domain listed there, on the far right, you'll see "Open in Control Panel".
6. Below the Domain, you'll see an arrow to expand more options. You'll see the "Virtual Directories" link, click it.
7. You'll see a "Mime Types" tab, click it and there you can add your mime type you need to use.