Klass API continues to support path extensions for content negotiation (#408)

* Add failing test

* Configure path extension content negotiation

- This is still an actively used API feature so it must be supported
- This is deprecated in Spring so it's likely to be removed in the future
- Limit the accepted file extensions for security

* Also support header content negotiation

* Correctly document content types for operations

* Correct content types for error responses

* API documentation covers all error formats

* Remove dot from file extensions

Author: mmwinther

klass-api/src/main/asciidoc/api-guide.adoc

@@ -0,0 +1,50 @@
+package no.ssb.klass.api.config;
+
+import no.ssb.klass.api.util.RestConstants;
+
+import org.springframework.context.annotation.Configuration;
+import org.springframework.http.MediaType;
+import org.springframework.web.accept.HeaderContentNegotiationStrategy;
+import org.springframework.web.accept.PathExtensionContentNegotiationStrategy;
+import org.springframework.web.servlet.config.annotation.ContentNegotiationConfigurer;
+import org.springframework.web.servlet.config.annotation.PathMatchConfigurer;
+import org.springframework.web.servlet.config.annotation.WebMvcConfigurer;
+
+import java.util.List;
+import java.util.Map;
+
+/**
+ * Prior to Spring Framework v3, path extension mapping was enabled by default.
+ *
+ * <p>This has been a central way for the Klass API to perform content negotiation and is currently
+ * in use by klass-web and presumably other clients.
+ *
+ * <p>This functionality is now deprecated in Spring and will be removed in a future release. We
+ * need to encourage clients to transition to header-based content negotiation if we want to keep
+ * pace with future Spring releases.
+ */
+@Configuration
+public class WebConfiguration implements WebMvcConfigurer {
+
+    @Override
+    public void configurePathMatch(PathMatchConfigurer configurer) {
+        configurer.setUseSuffixPatternMatch(true);
+        configurer.setUseRegisteredSuffixPatternMatch(true);
+    }
+
+    @Override
+    public void configureContentNegotiation(ContentNegotiationConfigurer configurer) {
+        configurer.strategies(
+                List.of(
+                        new HeaderContentNegotiationStrategy(),
+                        new PathExtensionContentNegotiationStrategy(
+                                // Only these path extensions are allowed.
+                                Map.of(
+                                        "csv",
+                                        MediaType.parseMediaType(RestConstants.CONTENT_TYPE_CSV),
+                                        "json",
+                                        MediaType.APPLICATION_JSON,
+                                        "xml",
+                                        MediaType.APPLICATION_XML))));
+    }
+}