[Feature]: Add filesystem config contentTypeCharset support (#2438)

* Update filesystem.go

* Update filesystem_test.go

* Update filesystem.md

* fmt

docs/api/middleware/filesystem.md

@@ -277,6 +277,12 @@ type Config struct {
     //
     // Optional. Default: ""
     NotFoundFile string `json:"not_found_file"`
+    
+    // The value for the Content-Type HTTP-header
+    // that is set on the file response
+    //
+    // Optional. Default: ""
+    ContentTypeCharset string `json:"content_type_charset"`
 }
 ```
 
@@ -290,5 +296,6 @@ var ConfigDefault = Config{
     Browse: false,
     Index:  "/index.html",
     MaxAge: 0,
+    ContentTypeCharset: "",
 }
 ```

middleware/filesystem/filesystem.go

@@ -53,6 +53,12 @@ type Config struct {
 	//
 	// Optional. Default: ""
 	NotFoundFile string `json:"not_found_file"`
+
+	// The value for the Content-Type HTTP-header
+	// that is set on the file response
+	//
+	// Optional. Default: ""
+	ContentTypeCharset string `json:"content_type_charset"`
 }
 
 // ConfigDefault is the default config
@@ -63,6 +69,7 @@ var ConfigDefault = Config{
 	Browse:             false,
 	Index:              "/index.html",
 	MaxAge:             0,
+	ContentTypeCharset: "",
 }
 
 // New creates a new middleware handler.
@@ -176,7 +183,11 @@ func New(config ...Config) fiber.Handler {
 		contentLength := int(stat.Size())
 
 		// Set Content Type header
+		if cfg.ContentTypeCharset == "" {
 			c.Type(getFileExtension(stat.Name()))
+		} else {
+			c.Type(getFileExtension(stat.Name()), cfg.ContentTypeCharset)
+		}
 
 		// Set Last Modified header
 		if !modTime.IsZero() {

middleware/filesystem/filesystem_test.go

@@ -218,3 +218,17 @@ func Test_FileSystem_UsingParam_NonFile(t *testing.T) {
 	utils.AssertEqual(t, nil, err)
 	utils.AssertEqual(t, 404, resp.StatusCode)
 }
+
+func Test_FileSystem_UsingContentTypeCharset(t *testing.T) {
+	t.Parallel()
+	app := fiber.New()
+	app.Use(New(Config{
+		Root:               http.Dir("../../.github/testdata/fs/index.html"),
+		ContentTypeCharset: "UTF-8",
+	}))
+
+	resp, err := app.Test(httptest.NewRequest(fiber.MethodGet, "/", nil))
+	utils.AssertEqual(t, nil, err)
+	utils.AssertEqual(t, 200, resp.StatusCode)
+	utils.AssertEqual(t, "text/html; charset=UTF-8", resp.Header.Get("Content-Type"))
+}