配置(#98): 扩展文件上传 Content-Type 白名单支持图片

- 在 etc/ppanel.yaml 与 internal/config/config.go 的 S3.AllowedContentTypes 新增 image/jpeg,image/jpg,image/png,image/webp,image/gif,image/heic,image/heif,image/bmp
- 保留原 zip/gzip/text/json/octet-stream
- validateInitRequest 在拒绝时携带 content_type is not allowed 业务消息
- 新增 internal/logic/public/file/common_test.go，覆盖允许/拒绝及无 Content-Type 嗅探
- doc/tapi-file-upload-zh.md 同步允许类型列表与错误码说明

Co-authored-by: multica-agent <github@multica.ai>

etc/ppanel.yaml

@@ -74,7 +74,7 @@ S3:
   UsePathStyle: false
   PresignExpireSeconds: 300
   MaxUploadSize: 104857600
-  AllowedContentTypes: "application/zip,application/x-zip-compressed,application/gzip,application/x-gzip,application/octet-stream,text/plain,application/json"
+  AllowedContentTypes: "application/zip,application/x-zip-compressed,application/gzip,application/x-gzip,application/octet-stream,text/plain,application/json,image/jpeg,image/jpg,image/png,image/webp,image/gif,image/heic,image/heif,image/bmp"
 
 device:
   enable: true # 开启设备加密通信