There is only one file type that uses the .X3F extension:
SIGMA X3F Camera RAW File
An X3F file is a RAW file saved in SIGMA's X3F format, a high-end digital photo format also known as the Foveon X3 format or "FOVb." It stores data for a single image captured with a digital camera. X3F files support image data, thumbnails, preview images, and text property information.
SIGMA is a developer of high-end digital cameras. The X3F format is supported by several SIGMA cameras that use the Foveon X3 sensor, including the SD series SLR cameras and the DP series compact cameras.
X3F files can be opened by several programs, which include SIGMA Photo Pro, Corel PaintShop Pro, and CyberLink PhotoDirector. Adobe Photoshop can also open X3F files but may not support X3F files created by newer models of SIGMA cameras, such as the dp Quattro series cameras.
Updated: January 25, 2019