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