WordPress sanitize_callback on all file data

I am not sure if this is a php or wordpress question, maybe both. I want to use the default wordpress file uploader and then run it through the customizer’s sanitize\_callback using my own script to then modify file content, all file types including images or non-text files. So basically I want to read all files as text and then modify the data. I can’t find much about this online with the words that I search for. How do I specify to read and look in the currently selected uploaded file? I am thinking it might be with the one code thing `$_FILES` but I am not too familiar with this in conjunction with wordpress. Thanks.


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