diff --git a/phpBB/docs/CHANGELOG.html b/phpBB/docs/CHANGELOG.html
index e54c4fd9bb..49c8d1364b 100644
--- a/phpBB/docs/CHANGELOG.html
+++ b/phpBB/docs/CHANGELOG.html
@@ -101,6 +101,7 @@
 		<li>[Fix] Minor language fixes. (Bug #54855)</li>
 		<li>[Fix] Parsing urls in signatures properly uses config settings. (Bug #57105)</li>
 		<li>[Fix] Allow multibyte keys in request_var(). (Bug #51555)</li>
+		<li>[Fix] Prevent wrong tar archive type detection. (Bug #12531)</li>
 		<li>[Feature] Support for Microsoft's Native SQL Server Driver for PHP (Bug #57055 - Patch by Chris Pucci at Microsoft)</li>
 	</ul>
 
diff --git a/phpBB/includes/functions_compress.php b/phpBB/includes/functions_compress.php
index f17c780a65..f422eaa8c1 100644
--- a/phpBB/includes/functions_compress.php
+++ b/phpBB/includes/functions_compress.php
@@ -502,8 +502,8 @@ class compress_tar extends compress
 	function compress_tar($mode, $file, $type = '')
 	{
 		$type = (!$type) ? $file : $type;
-		$this->isgz = (strpos($type, '.tar.gz') !== false || strpos($type, '.tgz') !== false) ? true : false;
-		$this->isbz = (strpos($type, '.tar.bz2') !== false) ? true : false;
+		$this->isgz = preg_match('#(\.tar\.gz|\.tgz)$#', $type);
+		$this->isbz = preg_match('#\.tar\.bz2$#', $type);
 
 		$this->mode = &$mode;
 		$this->file = &$file;