C3900-universalk9-mz.spa.157-3.m8.bin ★ Safe & Essential

The complex string of characters in the filename identifies the hardware compatibility, feature set, and versioning of the firmware: Cisco 3900 Series Integrated Services Routers

However, this image is suitable for:

Here is the proper breakdown:

Set as the boot variable using the boot system command in global configuration mode. Verification Checklist C3900-universalk9-mz.spa.157-3.m8.bin