If you have the starter kit, there's an app on there called FTBrony, which is a FTP server. It allows you to wirelessly transfer files. The n3DS already has something similar in the system settings, but it's less convienient and doesn't work on Linux or Mac. This works on both o3DS and n3DS as well. If you don't like the MLP theme, you can use FTP-3DS, which is the same functionality wise, but has a Nintendo theme instead.

 

 To use this, you need a device with an FTP client. I personally like use FileZilla, but anything will work. (Android has some as well. You can try this app.) Open the FTP server on your 3DS, and it will tell you your IP and port. Type that into your FTP client, and it will work as any other file manager would. If it doesn't connect at first, keep trying, and it should connect eventually.