Open "mGalaxy > Data > BaseSystems > BaseSystems.xml" with a text editor
Locate that block (search for "Sony PS1"
<System type="Console">
<Name ss="57" em="Sony_Playstation" tgdb="Sony Playstation" gb="22|PS1">Sony PS1</Name>
<Emu selected="ePSXe">
<Cmd hidden="false" name="ePSXe" value="-nogui -loadbin "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
<Cmd hidden="false" name="PCSX-Reloaded" value="-nogui -cdfile "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
<Cmd hidden="false" name="PSXFin" value="-f "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
<Cmd hidden="true" name="RetroArch [mednafen_psx_libretro.dll]" value="-f -L cores\mednafen_psx_libretro.dll "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
<Cmd hidden="true" name="RetroArch [mednafen_psx_hw_libretro.dll]" value="-f -L cores\mednafen_psx_hw_libretro.dll "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
<Cmd hidden="false" name="XEBRA [Script]" value="-IMAGE "%path\%file%ext" -RUN2 -FULL" extensions=".bin,.cue,.ecm,.img,.iso"/>
</Emu>
</System>
Edit this line
<Cmd hidden="false" name="ePSXe" value="-nogui -loadbin "%path\%file%ext"" extensions=".bin,.cue,.ecm,.img,.iso"/>
to this (.pbp added to the extensions list)
<Cmd hidden="false" name="ePSXe" value="-nogui -loadbin "%path\%file%ext"" extensions=".pbp,.bin,.cue,.ecm,.img,.iso"/>
Save
From now, mGalaxy will be able to see and use PBP files
NOTE: you'll have to create that systems again to benefit from the changes. Using the one you already created won't work!