diff --git a/client.go b/client.go
index f36231c..5c2fd33 100644
--- a/client.go
+++ b/client.go
@@ -517,7 +517,7 @@ func (c *Client) WebWxUploadMediaByChunk(ctx context.Context, file *os.File, opt
 	}
 
 	// 获取文件的类型
-	mediaType := getMessageType(filename)
+	mediaType := messageType(filename)
 
 	path, err := url.Parse(c.Domain.FileHost() + webwxuploadmedia)
 	if err != nil {
diff --git a/global.go b/global.go
index c671d7f..6ff5387 100644
--- a/global.go
+++ b/global.go
@@ -110,27 +110,27 @@ const (
 )
 
 const (
-	// 分块上传时每次上传的文件的大小
-	chunkSize int64 = (1 << 20) / 2 // 0.5m
-	// 需要检测的文件大小
-	needCheckSize int64 = 25 << 20 // nolint:unused
-	// 最大文件上传大小
-	maxFileUploadSize int64 = 50 << 20 // nolint:unused
-	// 最大图片上传大小
-	maxImageUploadSize int64 = 20 << 20 // nolint:unused
+	// 文件大小单位
+	_  = 1 << (10 * iota) // 1 << 0  = 1B
+	KB                    // 1 << 10 = 1KB
+	MB                    // 1 << 20 = 1MB
 )
 
-const TimeFormat = "Mon Jan 02 2006 15:04:05 GMT+0800 (中国标准时间)"
+const (
+	// ChunkSize 分块上传时每次上传的文件大小 (512KB)
+	chunkSize = 512 * KB
+
+	// needCheckSize 需要检测的文件大小 (25MB)
+	needCheckSize = 25 * MB // nolint:unused
 
-var imageType = map[string]struct{}{
-	"bmp":  {},
-	"png":  {},
-	"jpeg": {},
-	"jpg":  {},
-	"gif":  {},
-}
+	// maxFileUploadSize 最大文件上传大小 (50MB)
+	maxFileUploadSize = 50 * MB // nolint:unused
 
-const videoType = "mp4"
+	// maxImageUploadSize 最大图片上传大小 (20MB)
+	maxImageUploadSize = 20 * MB // nolint:unused
+)
+
+const TimeFormat = "Mon Jan 02 2006 15:04:05 GMT+0800 (中国标准时间)"
 
 // FileHelper 文件传输助手
 const FileHelper = "filehelper"
diff --git a/message.go b/message.go
index 0a07445..c4013d6 100644
--- a/message.go
+++ b/message.go
@@ -714,7 +714,7 @@ func newFileAppMessage(stat os.FileInfo, attachId string) *appmsg {
 	m.AppAttach.AttachId = attachId
 	m.AppAttach.TotalLen = stat.Size()
 	m.Type = 6
-	m.AppAttach.FileExt = getFileExt(stat.Name())
+	m.AppAttach.FileExt = fileExtension(stat.Name())
 	return m
 }
 
diff --git a/parser.go b/parser.go
index e3b7407..6ad6315 100644
--- a/parser.go
+++ b/parser.go
@@ -45,28 +45,33 @@ func GetFileContentType(file io.Reader) (string, error) {
 	return http.DetectContentType(data), nil
 }
 
-func getFileExt(name string) string {
-	ext := filepath.Ext(name)
+// fileExtension
+func fileExtension(name string) string {
+	ext := strings.ToLower(filepath.Ext(name))
 	if len(ext) == 0 {
-		ext = "undefined"
+		return "undefined"
 	}
 	return strings.TrimPrefix(ext, ".")
 }
 
-const (
-	pic   = "pic"
-	video = "video"
-	doc   = "doc"
-)
+// 判断是否是图片
+func isImageType(imageType string) bool {
+	switch imageType {
+	case "bmp", "png", "jpeg", "jpg", "gif":
+		return true
+	default:
+		return false
+	}
+}
 
 // 微信匹配文件类型策略
-func getMessageType(filename string) string {
-	ext := getFileExt(filename)
-	if _, ok := imageType[ext]; ok {
-		return pic
+func messageType(filename string) string {
+	ext := fileExtension(filename)
+	if isImageType(ext) {
+		return "pic"
 	}
-	if ext == videoType {
-		return video
+	if ext == "mp4" {
+		return "video"
 	}
-	return doc
+	return "doc"
 }