Add a comment that the 'type' format does not exist in OAS 3.0

Author: lion7

src/main/java/io/swagger/codegen/v3/generators/DefaultCodegenConfig.java

@@ -1024,7 +1024,7 @@ private static String getTypeOfSchema(Schema schema) {
         } else if (schema instanceof BinarySchema) {
             return SchemaTypeUtil.BINARY_FORMAT;
         } else if (schema instanceof FileSchema) {
-            return "file";
+            return "file"; // FIXME: this type does not exist in the OpenAPI 3.0 specification
         } else if (schema instanceof BooleanSchema) {
             return SchemaTypeUtil.BOOLEAN_TYPE;
         } else if (schema instanceof DateSchema) {