fix: 用硬编码MIME映射替代mime.TypeByExtension

- Alpine镜像缺少/etc/mime.types导致TypeByExtension不可靠
- 使用getContentType函数硬编码常见Web文件MIME类型
- 不再依赖系统环境，确保所有平台行为一致

backend/cmd/main.go

@@ -5,7 +5,6 @@ import (
 	"fmt"
 	"io/fs"
 	"log"
-	"mime"
 	"net/url"
 	"os"
 	"strings"
@@ -225,15 +224,6 @@ func startServer() {
 	r := gin.Default()
 	r.MaxMultipartMemory = 32 << 20
 
-	mime.AddExtensionType(".js", "application/javascript")
-	mime.AddExtensionType(".mjs", "application/javascript")
-	mime.AddExtensionType(".css", "text/css")
-	mime.AddExtensionType(".html", "text/html")
-	mime.AddExtensionType(".json", "application/json")
-	mime.AddExtensionType(".svg", "image/svg+xml")
-	mime.AddExtensionType(".woff", "font/woff")
-	mime.AddExtensionType(".woff2", "font/woff2")
-
 	r.Use(middleware.Logger())
 	r.Use(middleware.Recovery())
 	r.Use(middleware.Cors())
@@ -282,10 +272,7 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 			return
 		}
 
-		contentType := mime.TypeByExtension(decodedPath)
-		if contentType == "" {
-			contentType = "application/octet-stream"
-		}
+		contentType := getContentType(decodedPath)
 
 		c.Header("Cache-Control", "public, max-age=31536000, immutable")
 		c.Data(200, contentType, data)
@@ -303,10 +290,7 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 			decodedPath = path
 		}
 		if data, err := fs.ReadFile(distFS, decodedPath); err == nil {
-			contentType := mime.TypeByExtension(decodedPath)
-			if contentType == "" {
-				contentType = "application/octet-stream"
-			}
+			contentType := getContentType(decodedPath)
 			c.Data(200, contentType, data)
 			return
 		}
@@ -349,3 +333,43 @@ func getSettingString(settings map[string]interface{}, key, defaultValue string)
 	}
 	return defaultValue
 }
+
+func getContentType(filename string) string {
+	ext := strings.ToLower(filename[strings.LastIndex(filename, "."):])
+	switch ext {
+	case ".js", ".mjs":
+		return "application/javascript; charset=utf-8"
+	case ".css":
+		return "text/css; charset=utf-8"
+	case ".html":
+		return "text/html; charset=utf-8"
+	case ".json":
+		return "application/json; charset=utf-8"
+	case ".png":
+		return "image/png"
+	case ".jpg", ".jpeg":
+		return "image/jpeg"
+	case ".gif":
+		return "image/gif"
+	case ".svg":
+		return "image/svg+xml"
+	case ".ico":
+		return "image/x-icon"
+	case ".webp":
+		return "image/webp"
+	case ".woff":
+		return "font/woff"
+	case ".woff2":
+		return "font/woff2"
+	case ".ttf":
+		return "font/ttf"
+	case ".eot":
+		return "application/vnd.ms-fontobject"
+	case ".xml":
+		return "application/xml"
+	case ".txt":
+		return "text/plain; charset=utf-8"
+	default:
+		return "application/octet-stream"
+	}
+}