What I would like to be able to say is:
<http://totl.net/> dct:format <http://example.org/mimetype/text/html> .
<http://totl.net/> dct:format <http://example.org/encoding/utf-8> .
Of course I could just invent my own namespace, but I'd rather find the "right" one.
If there is no right one, there should be.
The NEPOMUK Information Element Ontology has a :mimeType predicate. Is has a xsd:string as range; the domain might be a problem, as it is defined as a :InformatationElement which may not be compatible with your application.
You shouldn't feel intimidated to create your own predicates in case you find that other people's predicates aren't perfect for your application.
You can always use some kind of inferencing (such as rdf:subPropertyOf) to map your crisp private predicate to 'vernacular' predicates that other people use. The big picture is that "anyone can say anything about any subject", and you shouldn't let a lack of the right predicate slow you down.