Is it good to store the uploaded file into a relational database or put it in a file system under a directory in IIS? I thought relational system will be a better choice. Any comments? When will you use one over the other?
EDIT: RDBMS, will make it easier to relate multiple file attachments to a record. It's easier to maintain version(s)
File systems make it easier to store data and I think performance wise this is a better choice.
It really depends on the case. If you are storing very large files your DB will become really big - consider this - it may affect the price of your hosting.
Storing files in a DB is preferred when you are in a web farm - for example if the requests to your application are processed by several servers and you store the files in a DB, then SQL clustering is all you need, and storing the files in the file system in such case is a lot harder - you have to use a common location or synchronize the files through the server farm!
I would use a DB for a file store - one location for all the data related to your application - easier to maintain and backup/restore!
Here is an article explaining how to store in a DB: http://www.dbazine.com/sql/sql-articles/charran5
Here is an interesting reading about SQL Server 2008 and the filestream feature: http://www.devx.com/dotnet/Article/40812