NandFiler

NandFiler is the name of an application that comes bundled with Nintendo DSi SDK’s, also known as the DSi Panda. This application allows you to change any value in hex in any file, with Japanese character support.

NandFiler's icon.

First surface

The existance of NandFiler and TwlNmenu was discovered at Gamescom 2009 when users by the name of Demo and Shadowchao started discussions about 2 development apps. Unfortunatly, both were unable to open the applications, because there was security walking around and the Nintendo DSi unit was hooked up to a monitor. Fast-forwarding to Febuary 11th, 2013, a collector by the name of LimboFunk mentioned a Nintendo DSi Development Unit, as well as the correct names for the applications. There wasn't much discussion after that, except for references, with no dumps being made. However, on September 2nd, 2017, a TWLSDK was leaked containing WMTestTool, RelayStation, modified versions of DS Download Play and Rev. 11364 of NandFiler and TwlNMenu.
Text from TwlNmenu on RGDWiki.

Rumours

Before there was proper documentation about NandFiler, there were rumours going around that NandFiler was a savefile importer/exporter for Nintendo DS cartridges and DSiWare, similar to SaveFileFiler on the Panda 3DS. This feature turned out to be included, but you have the ability to export or 'fill' any file on the SD Card.

Screenshots

What NandFiler looks like when you start it up.

Highlighting a random file from Flipnote Studio (stored on the NAND).

The hex editor itself.

NandFiler backing up a folder from an application. The folder gets extracted as a file to the SD Card.

The hex editor again, now with on-screen instructions by pressing SELECT.

The Export and 'Fill' options, which can only be used on things stored on the SD Card.