Thank you for contacting us, and I'm sorry for the issue you are experiencing. I'm not sure what could be happening here. I just tested the migration of data from mSecure 4.5 to mSecure 5 with both attachments and customer icons, and everything worked as it was supposed to. After the migration, all of the attachments and custom icons are displayed in the app, and when I tap on the attachments, the larger image is displayed.
The only thing I can think of to try is to have you reset your mSecure 5 account, so you can try to migrate your data again. The other option would be to try to restore from a recent email backup you may have access to. Do you have an email backup you can restore from? If not, do you still have mSecure 4 installed on your phone, and do you want to try resetting the account to migrate the data again?