refactor: minor changes and fixes

internal/format/format.go

@@ -4,6 +4,7 @@ import (
 	"encoding/json"
 	"fmt"
 	"path/filepath"
+	"strings"
 
 	"gopkg.in/yaml.v3"
 )
@@ -92,11 +93,11 @@ func Unmarshal(data []byte, v any, inFormat DataFormat) error {
 // both the standard default format (JSON) and any command line
 // change to that provided by options.
 func DataFormatFromFileExt(path string, defaultFmt DataFormat) DataFormat {
-	switch filepath.Ext(path) {
-	case ".json", ".JSON":
+	switch strings.TrimLeft(strings.ToLower(filepath.Ext(path)), ".") {
+	case JSON.String():
 		// The file is a JSON file
 		return JSON
-	case ".yaml", ".yml", ".YAML", ".YML":
+	case YAML.String(), "yml":
 		// The file is a YAML file
 		return YAML
 	}