#1684: This is a first commit toward support for importing RAW files from cameras.  The current test case (Canon 
S90) has this problem: gPhoto will return the thumbnail for the RAW image, but no EXIF.  When the CR2 file is 
copied to the local machine, it's possible to extract the EXIF from the embedded JPEG file, but not the EXIF 
thumbnail.  This mismatch kills dupe detection.