Decode BASE64 encoded ZIP file received by http

Hello,
My app receives a ZIP file as a BASE64 encoded text file. It´s just a 80k file with a .db file inside.
The file is encoded by a PHP web app using standard functions. I dont´t have access to this server, so I can't change anything. I just receive the file as is.
I save this file as zipfile.txt to DirDefaultExternal.
I need to know how I can decode this file back to zip.
I have no idea how can I do this, not even in my windows notebook.
I know how to open the ZIP in my app, after it is...

Decode BASE64 encoded ZIP file received by http

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