Here\'s the setup:
1 web server running a C# app to which my users (stored in a MySQL database on said server) authenticate.
1 file server
While this isn't directly applicable since you're using MySql but it's something to consider (might even be available for free by using sql server 2008 express). Sql Server 2008 offers Filestream support which lets you access files as if they are directly stored in the database but actually reside on a fileserver. Then information on the other posts can help you with getting it to the user.
FILESTREAM Storage in SQL Server 2008