Unfortunately this doesn't make any sense. Chroma would not want text/plain to map to the Bicep lexer. I think the initial mimetype is much more appropriate.

The product group around Azure Biceps does not use a special mimetype and will not submit it at IANA for the time being, as it can be taken from the linked discussion. Therefore the mimetype text/plain should be used.
As long as they are not registered at IANA, they are only used by chroma. However, no application (e.g. IIS, Tomcat, etc.) delivers these mimetypes used by chroma.

In that case the mime type should be removed. Chrome also uses the MIME type to determine which lexer to use, and text/plain should never resolve to anything but plain text.