😁优化上传文件重复时返回已有文件

Admin.NET/Admin.NET.Core/Service/File/SysFileService.cs

@@ -165,7 +165,7 @@ public class SysFileService : IDynamicApiController, ITransient
         if (file == null) throw Oops.Oh(ErrorCodeEnum.D8000);
 
         // 判断是否重复上传的文件
-        string? fileMd5 = null;
+        var fileMd5 = string.Empty;
         if (_uploadOptions.EnableMd5)
         {
             using var fileStream = file.OpenReadStream();