SharePoint Online Search Filters - Managed Metadata

I don't think you can add additional file types in Office365 like sharepoint server. Maybe you can try the spfx route to add custom refiners as mentioned in blog below.

http://thecollaborationcorner.com/2017/10/16/build-dynamic-sharepoint-search-experiences-with-refiners-and-paging-with-spfx-office-ui-fabric-and-pnp-js-library/