do you know what filesystem Windows Phone used by default? Were there differences between the different versions (Windows Phone 7, 8, 7.1 and Windows 10 Mobile)? Is it possible that it is NTFS, just like on Windows 7, 8, 8.1 and 10?
Windows phone 7
Two file systems are supported by Microsoft in Windows Phone 7: IMGFS and TexFAT. IMGFS was already in use by previous versions, and holds the actual system and operating system files. TexFAT is an extended version of the FAT file system capable of addressing files larger than 4GB, and is used to store user files. Microsoft has opted for a unified storage approach, which means that applications and/or users can not distinguish between files in local storage or on a memory card. While this increases simplicity, it also means that crucial files may be stored on a memory card - removing said card will cause the phone to be restricted to making emergency calls only."
Windows phone 8.1
The internal user store is formatted in NTFS file system format.
Unfortunately there is no information I could find of the Windows phone 10 - better off asking at https://forum.xda-developers.com/.