Altered File Extensions?!

Try to open the file with another tool to confirm that the file is corrupted.

If your evidence image has less than 5000 files in it, go to accessdata.com and download FTK v1.81.4. You will be able to use it in demo mode with an image that small.

If your image is greater than 5000 files, then download accessdata's FTK Imager. You can open the image with that and export the offending file(s) and change the extension and see if the file is corrupted.

Did your teacher explain about the first few bytes of the file being the file signature? if so, then open the offending file in a hex editor and find the first few bytes, then search for those bytes on Gary Kessler's file signature page here http//www.garykessler.net/library/file_sigs.html That will tell you the exact program the file uses and the correct extension.

Good luck

Just renaming a file to change it's extension will not corrupt a file. The file was probably already corrupted.

As Markg43 said you want to examine the file. I would suggest with a hex editor to see if the file looks valid - this will also test your knowledge of file structures, so another good exercise!

If you want to use a tool to definitively identify what types of files you are really working with, use the trial version of FI TOOLS. You can download it from http//www.forensicinnovations.com/fitools.html

Been notoriously cheap, I find that this has a very good value for money (FREEWARE)
TRid
http//mark0.net/soft-trid-e.html

The expandable database concept allows for continue betterings/enhancements.

BTW, what do you mean by

but if i change the file extension after copying the file, it will open, and be corrupt.

Open with what?
How?

Can you post a practical example/report?

jaclaz