fix: 修复Alpine上MIME类型检测失败

- fallbackMimeTypes 硬编码映射优先，不依赖系统 mime.types
- Docker 镜像安装 mailcap 包提供 /etc/mime.types
- 移除不可靠的 mime.AddExtensionType 方案

Dockerfile

@@ -33,7 +33,7 @@ RUN CGO_ENABLED=0 GOOS=linux go build -a -installsuffix cgo -o main ./cmd/main.g
 
 FROM alpine:latest
 
-RUN apk --no-cache add ca-certificates tzdata su-exec
+RUN apk --no-cache add ca-certificates tzdata su-exec mailcap
 
 RUN addgroup -g 1001 -S appgroup && \
     adduser -u 1001 -S appuser -G appgroup

backend/cmd/main.go

@@ -6,7 +6,6 @@ import (
 	"fmt"
 	"io/fs"
 	"log"
-	"mime"
 	"net/url"
 	"os"
 	"strings"
@@ -226,15 +225,6 @@ func startServer() {
 	r := gin.Default()
 	r.MaxMultipartMemory = 32 << 20
 
-	mime.AddExtensionType(".js", "application/javascript; charset=utf-8")
-	mime.AddExtensionType(".mjs", "application/javascript; charset=utf-8")
-	mime.AddExtensionType(".css", "text/css; charset=utf-8")
-	mime.AddExtensionType(".html", "text/html; charset=utf-8")
-	mime.AddExtensionType(".json", "application/json; charset=utf-8")
-	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())
@@ -284,10 +274,7 @@ func setupEmbeddedFrontend(r *gin.Engine) {
 			return
 		}
 
-		contentType := mime.TypeByExtension(decodedPath)
-		if contentType == "" {
-			contentType = "application/octet-stream"
-		}
+		contentType := getContentTypeByExtension(decodedPath)
 
 		c.Header("Cache-Control", "public, max-age=31536000, immutable")
 		c.Data(200, contentType, data)
@@ -305,10 +292,7 @@ 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 := getContentTypeByExtension(decodedPath)
 			c.Data(200, contentType, data)
 			return
 		}
@@ -343,6 +327,35 @@ func serveIndexHTML(c *gin.Context, indexHTML string) {
 	c.Data(200, "text/html; charset=utf-8", []byte(html))
 }
 
+var fallbackMimeTypes = map[string]string{
+	".js":   "application/javascript; charset=utf-8",
+	".mjs":  "application/javascript; charset=utf-8",
+	".css":  "text/css; charset=utf-8",
+	".html": "text/html; charset=utf-8",
+	".json": "application/json; charset=utf-8",
+	".svg":  "image/svg+xml",
+	".png":  "image/png",
+	".jpg":  "image/jpeg",
+	".jpeg": "image/jpeg",
+	".gif":  "image/gif",
+	".ico":  "image/x-icon",
+	".webp": "image/webp",
+	".woff": "font/woff",
+	".woff2": "font/woff2",
+	".ttf":  "font/ttf",
+	".eot":  "application/vnd.ms-fontobject",
+	".xml":  "application/xml",
+	".txt":  "text/plain; charset=utf-8",
+}
+
+func getContentTypeByExtension(filename string) string {
+	ext := strings.ToLower(filename[strings.LastIndex(filename, "."):])
+	if ct, ok := fallbackMimeTypes[ext]; ok {
+		return ct
+	}
+	return "application/octet-stream"
+}
+
 func getSettingString(settings map[string]interface{}, key, defaultValue string) string {
 	if val, ok := settings[key]; ok {
 		if str, ok := val.(string); ok && str != "" {