The BMP is the icon shown on the screen, the csv is the details of where the item is located and the GPX is a compiled version of the files. You need at least the BMP and CSV files for POILOADER to produce the GPI file which is then copied to your sat nav.
This is what worked for me and confirmed by gps-data-team. You need the following POI files:
1) bmp = display icon
2) gpx = garmin data file
Additional "nice-to-have" file(s) if available:
3) mp3 = audible warning (gps-data-team usually provides this file; otherwise, Garmin unit will just chime as you approach the POI, or just record your own mp3.
When you have the mp3 file(s), just include it in the folder with the bmp and gpx files and load it as normal with the POI Loader. It should upload automatically as well. I use the "express" setting.
For the audible warnings and icons to work they must have exactly the same name as the poi file and be in the same directory. Only the file extensions are different.