It's been quite a while since the last version, so right about now's a good time for an update: mediumgauge has released PSP Filer v5.7. The full changelog for the latest version is as follows:
o fixed a bug to be frozen while accessing UMD (depending on CFW's version).
o added a feature to scroll left to draw file names wider in deep folders.
o fixed a bug to have shown a wrong bit rate of VBR MP3 file.
o fixed a bug that some MP3 file cannot be played which has a large (>64KB) header.