file.close()

.gitignore

 .vs/*
+.vs/FidoReader/v17/.suo

FidoReader/Form1.cs

@@ -804,6 +804,7 @@ namespace FidoReader
 				byte[] fileBuffer = new byte[file.Length];
 				file.Read(fileBuffer, 0, fileBuffer.Length);
 				certBox.Text = Convert.ToHexString(fileBuffer);
+				file.Close();
 			}
         }
     }