If you would like to unlock the Extended_ROM part of the memory that is usually used for GSM operators’ customization files, just follow the bellow steps:

• In HKLM/System/Storage Manager/AutoLoad/TRUEFFS_DOC1 change DWORD Bootphase from 1 to 2
• In HKLM/System/StorageManager/Profiles/TRUEFFS_DOC1 change DWORD MountHidden from 1 to 0
• Perform a soft reset

Extneded_ROM in my device containes the following files:

Voices folder – collected.ini inside
BPVer_9810.CAB 30K
Bbpw.cab 4 KB
Config.txt 1 KB – instructions for installing the Extended_ROM content
ChgScutAttri.exe 89KB – changes the StartMenu shortcuts’ attributes
iw3in1_Artemis.ARM.CAB 3,44MB – Czech, Poslish and Hungarian UI language localization
MP_CVSDcpl_20060718.cab 96KB
MP_TT6_Voice05_WWE.cab 663KB – TomTom voice
MP_ttn.cab 5,46KB – TomTom 6 Navigator
PP_DefaultPage_WWE.CAB 32KB – default.htm for IE with HTC logem
PP_RemoveBTlnk.cab 2KB – removes the blutooth icon
sunnysoft.cab 7KB – managing utility for UI language localization

Total Extended_ROM size is 14,91MB
Above mentioned files size 9,6MB

EDIT – BE AWARE
Unlocking the Extended_ROM may cause some problems. I found out that after I have unlocked it, I cannot access the files nor I can delete them. Moreover, after hard reset the customization files are not loaded. You can find more info here.