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.
The quick and simple way to handle your files is to get a quality piece of software, such as reaConverter. This software is extremely efficient in managing a wide range of batch conversions. As you will soon realize, reaConverter will help you avoid spending countless hours trying to figure out how to convert multiple SFW files as once.
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