Improve handling of media types

[hackermd]

This PR introduces the following changes:

• Update media type of JPEG-LS from image/x-jls to image/jls
• Allow use of */* and other wildcard media types for retrieval of frames. This facilitates retrieval of frames if it's not known whether the frames are compressed (which would require image/* media type) or uncompressed (which would require application/octet-stream media type)
• Add unit tests for different combinations of media types and transfer syntaxes