OpenFileDialog always returns only the file name: It can't return the stream because it does not know what you are going to do with it: Open it as text or binary.
If you are using it with
this.openFileDialog1.Multiselect = true;
Then you need to loop through all names and open them yourself:
foreach (String file in openFileDialog1.FileNames)
{
FileStream fileStream = new FileStream(file, FileMode.Open);
...
}