Developer Mediumgauge has recently rolled out version 5.2 of the PSP Filer homebrew application. Among other updates, this build appears to have been optimized for use with custom firmware 4.xx. As for the rest of the changes since it last release (quoitng the readme):

* filer:
o added "GAME4XX" folder to re-order game icons. (added "GAMEnaa" folder : n=0-9, a=0-9/a-z/A-Z)
* picture viewer:
o changed circle button's function to toggle "make a picture to fit screen width" and "to fit screen height".
o binary editor:
o fixed a bug that a help screen was vanished when Filer finishes to read a large file.

Full details on installing and using PSP Filer 5.2 in your handheld are available in the file bundle's readme. Enjoy the download, and be sure to inform Mediumgauge of any bugs or issues you may encounter while using this application.