The extension indicates a 7-Zip compressed archive. This format is popular in file-sharing communities because:
These archives are a common vector for Trojans or "hidden" executables. Since users expect to see many files inside, they may overlook a malicious .exe file disguised as an image. Yummy_Girl.7z
In the context of "Yummy_Girl," this is a common naming pattern for: The extension indicates a 7-Zip compressed archive