Fix download file wrong content-type (#9825) (#9835)

* Fix download file wrong content-type

* change the error text to be more precise

* fix test

Co-authored-by: Lauris BH <lauris@nix.lv>

routers/repo/download.go

@@ -12,6 +12,7 @@ import (
 	"strings"
 
 	"code.gitea.io/gitea/modules/base"
+	"code.gitea.io/gitea/modules/charset"
 	"code.gitea.io/gitea/modules/context"
 	"code.gitea.io/gitea/modules/git"
 	"code.gitea.io/gitea/modules/lfs"
@@ -33,7 +34,12 @@ func ServeData(ctx *context.Context, name string, reader io.Reader) error {
 	name = strings.Replace(name, ",", " ", -1)
 
 	if base.IsTextFile(buf) || ctx.QueryBool("render") {
-		ctx.Resp.Header().Set("Content-Type", "text/plain; charset=utf-8")
+		cs, err := charset.DetectEncoding(buf)
+		if err != nil {
+			log.Error("Detect raw file %s charset failed: %v, using by default utf-8", name, err)
+			cs = "utf-8"
+		}
+		ctx.Resp.Header().Set("Content-Type", "text/plain; charset="+strings.ToLower(cs))
 	} else if base.IsImageFile(buf) || base.IsPDFFile(buf) {
 		ctx.Resp.Header().Set("Content-Disposition", fmt.Sprintf(`inline; filename="%s"`, name))
 	} else {