Your iOS device needs to be connected to the computer — you can't do this with Wi-Fi syncing. And the next time you sync the device, iTunes will delete any content you've added with Waltr, because it's not in your iTunes library.
I'd love to see Waltr go further, and convert files and add them to your iTunes library, if you want. I use different apps to convert audio and video files, and it can get a bit complicated, especially for people who don't do this often. This would make the app a one-stop shop for most people's audio and video file conversion needs. But Waltr is an excellent tool to get media files onto your iOS device quickly, and to no longer worry about unsupported formats.
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