I know...
They are
Second Nature image files.
SCN =
Scene files.
See the Second Nature web site for some really wonderful images...
http://www.secondnaturecd.com/I have thousands of them.
They are a sub set of JPEG that Second Nature uses. I believe they are used only by Second Nature but may be used by others on digital photogrphy to denote a Scene file.
The main difference between JPEG, (or JPG) & SCN files is that the SCN files can link up to a text file for artist bios & other information relating to the image file.
With each set of Second Nature files that you download (usually in sets of 20 - 24) there is one special additional file (Thumbnails.cat) with all of the artist information for the SCN files to link up to; works very well.
MC does not currently support the SCN format or extension but I wish they would. I use ACDSee as it does see full functionally of SCN files.
As I don't really care for the artist information, I usually just use ACDSee to batch convert all of my SCN files to standard JPG files that program (including MC) can read.
Hope this helps