This overrides the normal content/extension detection, but can be overridden
by the user.

This is an input plugin API extension; ABI is unchanged.
The version is not bumped (we can't bump it due to 1.2).

text/plain is specifically filtered out because it could cause lots of false
positives, at least with http; web servers tend to default to text/plain for
unknown types.

These occur where the MIME type used as the key is a substring of a type in
some plugin's MIME type list but is not an exact match.

Now that the macro are either imported from the system or defined by
configure. don't define them in every source file.

All the initialisation functions returning a new object instance that
was allocated through malloc() or calloc() can get the malloc
attribute so that the compiler can optimise their call.