Rails.root returns a Pathname object. Pathname#+(string) will File.join string to the path, if it is relative; if string is an absolute path (i.e. begins with a slash), the path is replaced.
Pathname.new('/tmp') + 'foo'
This means that you are reading the wrong way: you wanted to read /path/to/app/public/albums/files.zip , but in fact you are reading /public/albums/files.zip , which probably should not exist.
Solution: make sure you add the relative path:
Rails.root + 'public/albums/files.zip'
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