title: Are XLSX files UTF-8 encoded by definition?
body: <p>I'm trying to read in XLSX files with PHP. Using <a href="https://github.com/gneustaetter/XLSXReader" rel="noreferrer">gneustaetter/XLSXReader</a> to be exact. However, these XLSX-files are generated by different companies, using different software. So I wanted to check if they have the right encoding and always just found UTF-8.</p>
  
  <p>Therefore my question as above: 
  Are XLSX files UTF-8 encoded by definition? Or are there exceptions that could break the import script I'm working on?</p>