The SFW format is associated with Seattle Film Works, a company which used to specialize in processing photographic films by mail order. SFW files most likely contain digital versions of images processed by this company. Seattle Film Works gained a lot of popularity in the United States during the 1990s, but was closed in early 2000s. In case you found an SFW file, it probably represents an image that dates from that time, and might be difficult to open using regular image viewers.
JPEG JPG PNG GIF TIFF TIF BMP ICO EMF WMF AI PDF EPS PS EPI WEBP WEBPLL JP2 PCX TGA DWG DWF DXF DGN WMZ CGM DRW GBR PCT MIF PLT PCL SVG WPG CIN DCX DIS HRU MTV PRC RAD PIC RLA QRT SGI XBM XPM SWF CAL PSD JXR BPG APNG DDS SVGZ CG4 FLIF