I had some trouble with filetypes, the deficons were not displayed. By adding 'PatternHint' to the filetype it works. Here's an example:

AMTD
1
Type audio/mp3
Match
PatternHint #?.mp3
Name #?.mp3
End
Action
Name Play with AmiNetRadio
Event DoubleClick
Command AMIGADOS rx Sys:S/ANR.rexx url %sp
End

End

Don't know if this is the correct way but it seems to work for me.