The default set of supported mime-types is kept small because we want to make sure the runtime does not do any unnecessary processing. If the mime type your application is using is not in the default set you have 2 options to enable more mime types.
If you have the source of the application this is the preferred way. Add a <mime-type> block defining the mime type you want added to your web.xml and it will be added to the MimeTypeManager.
If you do not have the source you can create a WebApplicationExtension that calls the MimeTypeManager API to add the mime type programmatically. And then if you are using Piranha Micro you can use the --extension on the command line to add it.