Your file will not be uploaded to the server, nor will any data be sent to the server.
Disclaimer: The preview might not be 100% accurate. For the best experience, you should try the song out within the main application.
However, I do appreciate it if you accept some cookies and sessions, as they allow me to actually hide this notification. I might "require" it for other things that are specific to just this site, but I will guarantee that if you do accept these cookies that I will not use any data that I might be able to collect, mostly because I don't know how to collect this data, and I really have no use for it.
Also I need to set up a session so that this notification doesn't appear again in the next thirty days or so. This isn't to pressure you into using cookies, it's just that cookies are the only way to reliably remember that you don't want to see this notifications bar again, though I've tested this site for mobile use, and the bar isn't really that annoying.
Regardless of what you choose, if you accept any cookies and / or data, they will expire thirty days after you last logged in.
So, in short:
Stores site-specific cookies and sessions
Stores cookies and sessions from external services