OK Harry, seems like you changed the extension of an image file (.DT2) to .txt then tried to get Notepad to open it, plain text editors like Notepad cannot open image files.
So we'll try something slightly different.
Open the folder which contains the original .DT2 files. Click to open any .DT2 file and when the pane opens saying that Windows cannot open this file, check the box to select from a list of files then click OK. A window showing a list of well-known programs will be displayed, on that list highlight Internet Explorer (or Firefox if that is your browser) then check the box which says "Always use the selected program to open this kind of file" then click OK. Alternatives to Internet Explorer or Firefox could be any image displaying program such as Windows Picture and Fax Viewer or another image program you have installed, such as Irfanview.
From here on any time you click on a .DT2 file whichever program you selected will open the file. The hip name for this process is File Association, in other words you have associated the file type (DT2) with a program to open it.
Do the same for the .ID2 files but this time select Notepad as the program to open them.
Good luck