*.nds files are the only files your R4 can natively launch, as those are DS binaries. However, the *.nds files can access other files on your system. For instance, you can launch Moonshell.nds which can access the *.mp3 and *.jpg and *.txt files on your card (and other file formats as well).
*.gba files are made for the GBA, and they won't run on any slot-1 device like an R4 without a slot-2 device of some sort.
*.nfo files are just information files. Open them with Notepad to see what they say.
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