Description:
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PharFileInfo::decompress marks the file in the phar as not compressed but when you extract the file from the phar it will be compressed.

Test script:
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<?php
file_put_contents("test.txt", "This is a test file.");
$phar = new Phar("test.phar");
$phar->addFile("test.txt");
$file = $phar["test.txt"];
var_dump($file->compress(Phar::GZ)); //true (success)
var_dump($file->isCompressed()); //true (the file is compressed)
var_dump($file->decompress()); //true (success)
var_dump($file->isCompressed()); //false (the file should not be compressed anymore)
var_dump($phar->extractTo("test")); //true (success but the extracted file in the folder is compressed)