judging by the adresses you used i guess you found them in ram? so, not possible to find via IDA?
Of course, I using IDA for this:
(a710)
some_f_for_dng = 0x89E8
0x89E8 + 0xFFF13950(canon_data_src from boot.c) - 0x1900 = 0xFFF1AA38 (from here pointer to function (sub_FFE4C390) copied to RAM address 0x89E8 at camera/CHDK startup).
second_ext_for_dng = 0x8A0C
0x8A0C + 0xFFF13950(canon_data_src from boot.c) - 0x1900 = 0xFFF1AA5C (from here pointer to '.THM' copied to RAM address 0x8A0C at camera/CHDK startup).
But no all extension can be replace by 'dng'. Easiest case if 'cr2' files are visible via USB (for a710 'cr2' files are not visible).
P.S. For example, for a710 list of file extensions which camera scans in different folders at USB connection, located at 0xFFE68580 (in ROM) - jpg, crw, avi, wav, mrk, dps, fir.
Second list of extensions which can be transfred via USB located at 0xFFF1AA48 (and copied to RAM at startup) - fir, mrk, jpg, crw, avi, thm, wav. You can see that these lists are different.