How can I get the assembly file version

There are three versions: assembly, file, and product. They are used by different features and take on different default values if you don't explicit specify them.

string assemblyVersion = Assembly.GetExecutingAssembly().GetName().Version.ToString(); 
string assemblyVersion = Assembly.LoadFile("your assembly file").GetName().Version.ToString(); 
string fileVersion = FileVersionInfo.GetVersionInfo(Assembly.GetExecutingAssembly().Location).FileVersion; 
string productVersion = FileVersionInfo.GetVersionInfo(Assembly.GetExecutingAssembly().Location).ProductVersion;

See my comment above asking for clarification on what you really want. Hopefully this is it:

System.Reflection.Assembly assembly = System.Reflection.Assembly.GetExecutingAssembly();
System.Diagnostics.FileVersionInfo fvi = System.Diagnostics.FileVersionInfo.GetVersionInfo(assembly.Location);
string version = fvi.FileVersion;