Add check for Opus file type

Signed-off-by: Mahyar Koshkouei <deltabeard@users.noreply.github.com>

source/opus.c

@@ -1,6 +1,16 @@
 #include <3ds.h>
+#include <opus/opusfile.h>
 
 int playOpus(const char* in)
 {
 	return 0;
 }
+
+int isOpus(const char* in)
+{
+	int err = 0;
+	OggOpusFile* opusTest = op_test_file(in, &err);
+
+	op_free(opusTest);
+	return err;
+}