As an email administrator I am frequently forwarded emails with file attachments that the recipient can't open for various reasons. Perhaps is a proprietary format and the users doesn't have the software. Perhaps it is corrupted, perhaps the sender sent the wrong thing in the first place.
When I don't know what the heck file extension I am working with I usually resort to google-fu skills. I recently discovered a handy web site that lists information about various file types, who the software manufacturer is who has that type reserved, and what software is needed to access the files.
File-extension-library.com is a handy reference you might need someday. Here for example is their page about the jpg file type - file extension jpg.
They very helpfully let you know that jpgs are lossy (compressed and able to lose quality) image files that can be viewed in a web browser and edited with various image editing software.
