File naming extensions are often hidden as default in Windows to protect users when renaming files from loosing data or system funcationality.  Conversely malware authors may create a file such as PHOTO.JPG.EXE (where the EXE is hidden and it can appear a file usually perceived to be “safe”. 

First link below shares how to set or unset the “Show Hidden File Name Extensions” option.  I usually always like to show them on both home and work PCs, and am very careful when renaming items.  First link is excellent and approach can be used in flexible manner as desired

QUOTE: Another trick is the double extension, which takes advantage of Windows’ file-naming conventions. If a file is named adorable.jpg.exe, most Windows computers will display it as adorable.jpg. Most users, therefore, will think it a harmless image file, even though it’s really an executable program. And when you run the program, it probably will show you an adorable picture…while it infects your PC