Refine String to File conversion support

Refine the conversion support so that file URLs are support without
reintroducing gh-12163.

Closes gh-16931

spring-boot-project/spring-boot/src/main/java/org/springframework/boot/convert/StringToFileConverter.java

@@ -41,7 +41,7 @@ class StringToFileConverter implements Converter<String, File> {
 			return getFile(resourceLoader.getResource(source));
 		}
 		File file = new File(source);
-		if (file.isAbsolute()) {
+		if (file.exists()) {
 			return file;
 		}
 		Resource resource = resourceLoader.getResource(source);