Identifies which content types are compressable. In general, content types are assumed to be compressable. The
configuration of the service identifies exceptions, which are usually image file formats.
The configuration maps content types to boolean values (true for compressable).
Since 5.4, the contributed values may also be a wild-card such as "image/*" (that is, the subtype
may be a '*' to match any content type with the same top-level type).