Storing temporary user files in ASP.NET in medium trust

I have a scenario where users of my ASP.NET web application submit testimonials consisting of text info and images. The submit process has the following steps:

First the user inputs the content and chooses a path to an image
When he clicks preview, the info is once again shown so that he can confirm
Once confirmed the info is persisted in the database

The problem with this is that I don't want to store uploaded images in the DB before the user actually confirms. Instead I store them as temporary files and put them in DB only after final confirmation.

Since I also want my application to run in medium trust, I have write permissions only to the application directory and nowhere outside. I even want to limit write permissions for the ASPNET / NETWORK SERVICE user to the ~/App_Data folder. The problem with my scenario is that once a temporary file is created in this folder, the application pool is recycled and I don't want that on every testimonial submit.

How do you advise I keep these temp files instead? The pool is not restarted if I update a file - only on create or rename. But I don't think I can store whole images in a single file for all users. What do you think?

UPDATE: I should note that I'm using a third party control for upload. It gives me programmatic access to the binary stream of the file contents after upload, but I cannot keep this after a second postback (the first step and postback actually does the upload).

Solution

I would recommend IsolatedStorage. It's a kind of virtual folder.

Here is an excerpt from an example on CodeProject:

IsolatedStorageFileStream stream = 
  new IsolatedStorageFileStream(ISOLATED_FILE_NAME, 
  FileMode.Create, isoStore);

StreamWriter writer = new StreamWriter( stream );
writer.WriteLine( "This is my first line in the isolated storage file." );
writer.WriteLine( "This is second line." );
writer.Close();

UPDATE: To clean up your file just do this:

string fileName = "isolatestorage.txt";

IsolatedStorageFile storage = IsolatedStorageFile.GetStore(
    IsolatedStorageScope.User | IsolatedStorageScope.Assembly, null, null);

string[] files = storage.GetFileNames(fileName);
foreach(string file in files) {
    if(file == fileName) {
        storage.DeleteFile(file);
        break;
    }
}