AnyImageConverter: bump plugin interface.

Can't really implement extension() and mimeType() here.

src/MagnumPlugins/AnyImageConverter/AnyImageConverter.cpp

@@ -626,4 +626,4 @@ bool AnyImageConverter::doConvertToFile(const Containers::ArrayView<const Compre
 }}
 
 CORRADE_PLUGIN_REGISTER(AnyImageConverter, Magnum::Trade::AnyImageConverter,
-    "cz.mosra.magnum.Trade.AbstractImageConverter/0.3.2")
+    "cz.mosra.magnum.Trade.AbstractImageConverter/0.3.3")

src/MagnumPlugins/AnyImageConverter/AnyImageConverter.h

@@ -121,6 +121,10 @@ extension and a corresponding plugin is loaded. After that, flags set via
 the concrete implementation, with a warning emitted in case given option is not
 present in the default configuration of the target plugin.
 
+The @ref extension() and @ref mimeType() functions can't be implemented as
+they depend on the plugin chosen inside @ref convertToFile(). Both return an
+empty string.
+
 The output of the @ref convertToFile() function called on the concrete
 implementation is then proxied back.
 */