Files with the FW.PNG extension are native PNG image files that were created using the Adobe Fireworks raster image and vector graphics editor.

Also similar to the standard PNG format, these files also contain layer data etc. These files are generally 'flattened' before being saved in the .PNG format.

Due to the patent-free nature of the PNG format, the reduced size of PNG image files and the ability to also contain non-image data, Adobe developers chose it as the native format for their Fireworks application. However, this can easily lead to confusion and even loss of data if users also choose the PNG format for their output files. Because both native files and output files share the same extension, it's easy to mistake and delete source files by accident or to overwrite them with compressed PNGs losing all the source data in the process.

Adobe Fireworks users often take the "double extension" approach to solve this problem and add ".FW" before ".PNG" to the name of their source files.