List is perpetually "null" (ASP MVC3) - asp.net-mvc

Question

I'm working on my first project using ASP MVC3. My application is code first using entity framework 4.1 and SQL server CE.
The application is a document library. The main model is Document which is a path to a pdf file and a bunch of metadata.
I have a "replace" function in my controller that takes one Document record, moves the file to an archive location, and updates the database with information about the replacement.
I am trying to store a list of strings with the document that represent filepaths to older versions of the same document.
No matter what I cannot get this list, called ArchivedFilePaths, to be anything but "null".
Model:
using System;
using System.Collections.Generic;
using System.Linq;
using System.Web;
using System.ComponentModel;
using System.ComponentModel.DataAnnotations;
using System.Web.Mvc;
namespace DocLibrary.Models
{
public class Document
{
public int DocumentId { get; set; }
public int CategoryId { get; set; }
public string DocumentCode { get; set; }
public string Title { get; set; }
public string Description { get; set; }
public string FileUrl { get; set; }
public int FileSize { get; set; }
public string FileSizeString { get; set; }
public int Pages { get; set; }
public string Creator { get; set; }
public int Revision { get; set; }
public DateTime CreatedDate { get; set; }
public DateTime LastModifiedDate { get; set; }
public virtual Category Category { get; set; }
public List<String> ArchivedFilePaths { get; set; }
public SoftwareVersion SoftwareVersion { get; set; }
}
Controller:
public ActionResult Replace(HttpPostedFileBase file)
{
if (file != null && file.ContentLength > 0)
{
int id = Int32.Parse(Request["DocumentId"]);
Document doc = docsDB.Documents.Find(id);
//move the current version to the ~/Archive folder
string current_path = Path.Combine(Server.MapPath("~/Files"), Path.GetFileName(doc.FileUrl));
string archiveFileName = Path.GetFileNameWithoutExtension(doc.FileUrl) + "-" + doc.Revision.ToString() + ".pdf";
string destination_path = Path.Combine(Server.MapPath("~/Archive"), archiveFileName);
System.IO.File.Move(current_path, destination_path);
if (doc.ArchivedFilePaths == null)
{
doc.ArchivedFilePaths = new List<String>();
}
doc.ArchivedFilePaths.Add(destination_path);
//there are a bunch of statements that update the title, number of pages, etc. here, all of these work fine
try
{
docsDB.Entry(doc).State = EntityState.Modified;
docsDB.Logs.Add(new Log { LogDate = DateTime.Now, LogText = "Document replaced with a new version: " + doc.Title, });
docsDB.SaveChanges();
return RedirectToAction("Index");
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
}
}
}
//if a file was not selected;
return RedirectToAction("Index");
}
The Index view displays all of the files and their properties, including ArchivedFilePaths. After replacing a document with a new file, all of the items in the Document model update properly except ArchivedFilePaths.
If I inspect the list in VisualStudio, it is not null after the doc.ArchivedFilePaths.Add statement. So I don't believe the list is ever saved in the database and I suspect there is something wrong with my model. If I change it to a single string I can update it just fine.
Does anyone have any insight? Thanks.