well a nice improvement you can do is:
1) add an absolute path as appSetting to your application web config, something like: \\MachineName\ShareName
2) create the ShareName network share on the machine MachineName (this could be your server where you publish the application or another server as well, up to your needs)
3) allow the IIS user or the application pool's identity user to access in read-write to the share: \\MachineName\ShareName
4) edit your code above to do something like:
Path.Combine(sharePath, filename);
where sharePath is a string initialized with the value from your web.config's appSetting using ConfigurationManager.AppSetting["settingName"]