go: add os import in case of File return type (#3996)

Author: bonifaido

modules/openapi-generator/src/main/java/org/openapitools/codegen/languages/AbstractGoCodegen.java

@@ -377,6 +377,11 @@ public Map<String, Object> postProcessOperationsWithModels(Map<String, Object> o
         boolean addedOSImport = false;
         boolean addedReflectImport = false;
         for (CodegenOperation operation : operations) {
+            // import "os" if the operation uses files
+            if (!addedOSImport && "*os.File".equals(operation.returnType)) {
+                imports.add(createMapping("import", "os"));
+                addedOSImport = true;
+            }
             for (CodegenParameter param : operation.allParams) {
                 // import "os" if the operation uses files
                 if (!addedOSImport && "*os.File".equals(param.dataType)) {