Revise Validation for ValueSet Mime Types

[alexanderkiel]

Currently the ValueSet Mime Types is validated using the expanded Codesystem urn:ietf:bcp:13. However the ValueSet description says:

This value set includes all possible codes from BCP-13 (http://tools.ietf.org/html/bcp13)

and at the expansion the reason

This value set cannot be expanded because of the way it is defined - it has an infinite number of members

is given to explain why there is no expansion.

IHMO, the ValueSet shouldn't be validated by comparing against the list of currently defined media types, it should be validated against a regular expression of all possible media types.

Background: we like to use a custom, not registered, media type for a Library resource attachment.