Very similar to how MySpace done it all those years back....
MySpace did it differently back then because they got sued for copyright violations for the music their users were uploading and streaming. https://www.google.com/search?q=myspace+lawsuit+music
These days, you can upload music to MySpace, but they have licenses and agreements with music publishers, and are clear to their users what can be legally uploaded: https://help.myspace.com/hc/en-us/articles/202591770-Music-Upload-Requirements
And remember.... By uploading any content to Myspace, you hereby
certify that you own the copyright in or have all the necessary rights
related to such content to upload it. Don't step on someone else's
And Myspace is are clear on how they handle copyright violations: https://help.myspace.com/hc/en-us/articles/202055394-Copyright-FAQs
Myspace will respond to allegations of copyright violations in
accordance with the Digital Millennium Copyright Act (“DMCA”). The
DMCA provides a process for a copyright owner to give notification to
an online service provider concerning alleged copyright infringement.
Now, with your question: you are allowing users to both upload music and stream it.
Streaming is legal in the UK; see an older question here on LSE Is Streaming Copyrighted Content Illegal in the UK
But the hosting - storing the actual files - of copyrighted material is not legal.
But my app knows no detail of the song uploaded. Could be anything....
This doesn't matter; the files are on your server.
You run the risk of legal action being taken against you for the actual .mp3 file of copyrighted music on your server. You are the owner of the webserver, and thus control the files on it, along with your knowledge of building an App that allows users to upload the files. Even if you claim to know nothing about what users upload with the App you built, the music files are on your server.
What happens also depends on where the server with the music files is located, i.e. in the UK (action against you legally possible), or elsewhere (legal action against depends on the country and their laws and agreements with other countries).
If your server is in a third-party country and can't be taken down, but you are a UK resident, the music companies can still go after you in other respects, as the paying owner of the server, the owner of the domain registration, even possibly as the creator of the App itself.