Hello, 


Looks like things aren't moving that fast on the exiv2 bug so I looked into the exiftool code and found out how the file number is encoded: in brain dead way: ( at least on my canon 400D)

in file : http://cpansearch.perl.org/src/EXIFTOOL/Image-ExifTool-8.60/lib/Image/ExifTool/Canon.pm
look for string "# File number information (MakerNotes tag 0x93)"

I hacked rapid-photo-downloader to implement this and you might be interested in integrating my ideas "your way".

Here is what I did:

in file metadataphoto.py: Here is my version of the shutter_count method ( bit shuffling to come with a 7 figures number:)

TODO: guard those changes  with a check on the camera model number ( see exiftool code) ( maybe those shuffling should be in exvi2)

    def shutter_count(self,  missing=''):
        try:
            keys = self.exif_keys
            if 'Exif.Nikon3.ShutterCount' in keys:
                v = self['Exif.Nikon3.ShutterCount'].raw_value
            elif 'Exif.CanonFi.FileNumber' in keys:
                v = self['Exif.CanonFi.FileNumber'].raw_value
                v = int(v)
                d = (v & 0xffc00) >> 10
                while d < 100:
                  d += 0x40
                v =  d*10000 + ((v & 0x3ff ) << 4 ) + ((v >> 20) & 0x0f);
            elif 'Exif.Canon.FileNumber' in keys:
                v = self['Exif.Canon.FileNumber'].raw_value
            elif 'Exif.Canon.ImageNumber' in keys:
                v = self['Exif.Canon.ImageNumber'].raw_value
            else:
                return missing
            return str(v)
        except:
            return missing


in file generatenameconfig.py I added two sub choices for formatting the shutter_count value

TODO: create values specific to this use case: DCIM_DIRECTORY_NUMBER DCIM_IMAGE_NUMBER ? 

LIST_SHUTTER_COUNT_L2 = [
                     SEQUENCE_NUMBER_3,
                     SEQUENCE_NUMBER_4,
                     SEQUENCE_NUMBER_5,
                     SEQUENCE_NUMBER_6,
                     SUBFOLDER_SEQ_NUMBER,
                     STORED_SEQ_NUMBER,
                     ]

in file generatename.py in method _get_metadata_component

        elif self.L1 == SHUTTER_COUNT:
            v = self.rpd_file.metadata.shutter_count()
            if self.L2 in  [ SUBFOLDER_SEQ_NUMBER, STORED_SEQ_NUMBER ] :
                if self.L2 ==  SUBFOLDER_SEQ_NUMBER :
                    v = str(v)
                    v = v[:-4]
                if self.L2 == STORED_SEQ_NUMBER :
                    v = str(v)
                    v = v[-4:]
            else:
                if v:
                    v = int(v)
                    padding = LIST_SHUTTER_COUNT_L2.index(self.L2) + 3
                    formatter = '%0' + str(padding) + "i"
                    v = formatter % v
        elif self.L1 == OWNER_NAME:
            v = self.rpd_file.metadata.owner_name()
  

Those patches work for my Canon 400D ( and also for a old Canon A85 compact camera).

If theses changes are more or less in line with your "roadmap", I can put a real patch together.

I don't have a lot of DSLR so it's difficult for me to do regression tests ...

With those changes I can configure rapid-photo-downloader according to my needs:
I can extract the DCIM directory name and the file name from the image metadata.