If you answer with "yes" to any of these questions, Decoupled wasn't made for you. Sorry.
However, if you...
Well, you have several options:
There's one important thing though: due to limitations of the iOS platform, when transferring files via iTunes File Sharing, the newly transferred items will appear only when you tap the refresh button at the top, or the next time the app is launched.
First you should open the in-app file manager ("Files"), and from there open the "Transfer" page.
Now, you should choose whether you want to use an FTP client (e.g. FileZilla on macOS or Total Commander on Windows), or a WebDAV client (e.g. Cyberduck both for macOS and Windows) for transferring files. If you choose WebDAV, you can use a simple Web browser too, like Google Chrome.
Make sure you're connected to the same WiFi network as your computer, and press "Start server". This fires up a local FTP/WebDAV server on your phone, which you can access from your computer.
Once the server is running, you should take the displayed URL, and enter it into your FTP/WebDAV client or Web browser:
I recommend using Google Chrome, because that's the only Web browser that supports drag'n'drop for folders. Other browsers like Safari won't be able to import all the files in a directory tree.
Neither the WebDAV, nor the FTP server is password-protected. The FTP user is anonymous.
If you're not using macOS, you might not be able to use the user-friendly http://my-iphone.local URL, unless you have Bonjour or some other Multicast DNS client avilable (iTunes for Windows comes with a Multicast DNS client). In this case you should use the plain IP URL, displayed below the user-friendly one, e.g. http://192.168.0.220:58080
Put a file called "folder.jpg", "artwork.jpg", "album_art.jpg", "cover.jpg" or "front.jpg" next to the music files, and it will be used as the artwork for the given album. (PNG and GIF files are also supported.)
Artworks embedded in ID3 tags are also supported for MP3 files.
Swipe up on the playback screen.
Decoupled was made to be a standalone application, and not a companion to the iOS music app. Therefore, playing music from the iOS music library is not possible.
Decoupled is a hobby project, but if you have a feature idea that's currently missing from the app, and doesn't go against its philosophy, drop a mail to features at decoupled.app and I'll consider implementing it.
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