Fail fast if spring.config.location uses unknown file extension

Closes gh-17241

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/context/config/ConfigFileApplicationListener.java

@@ -453,6 +453,8 @@ public class ConfigFileApplicationListener implements EnvironmentPostProcessor,
 						return;
 					}
 				}
+				throw new IllegalStateException("File extension of config file location '" + location
+						+ "' is not known to any PropertySourceLoader");
 			}
 			Set<String> processed = new HashSet<>();
 			for (PropertySourceLoader loader : this.propertySourceLoaders) {

spring-boot-project/spring-boot/src/test/java/org/springframework/boot/context/config/ConfigFileApplicationListenerTests.java

@@ -65,6 +65,7 @@ import org.springframework.test.util.ReflectionTestUtils;
 import org.springframework.util.StringUtils;
 
 import static org.assertj.core.api.Assertions.assertThat;
+import static org.assertj.core.api.Assertions.assertThatIllegalStateException;
 
 /**
  * Tests for {@link ConfigFileApplicationListener}.
@@ -981,6 +982,16 @@ class ConfigFileApplicationListenerTests {
 		assertThat(this.environment.getProperty("gh17001loaded")).isEqualTo("true");
 	}
 
+	@Test
+	void whenConfigLocationSpecifiesUnknownFileExtensionConfigFileProcessingFailsFast() {
+		String location = "classpath:application.unknown";
+		TestPropertySourceUtils.addInlinedPropertiesToEnvironment(this.environment,
+				"spring.config.location=" + location);
+		assertThatIllegalStateException()
+				.isThrownBy(() -> this.initializer.postProcessEnvironment(this.environment, this.application))
+				.withMessageContaining(location);
+	}
+
 	private Condition<ConfigurableEnvironment> matchingPropertySource(final String sourceName) {
 		return new Condition<ConfigurableEnvironment>("environment containing property source " + sourceName) {