Index: includes/media/Bitmap.php
===================================================================
--- includes/media/Bitmap.php	(revision 95588)
+++ includes/media/Bitmap.php	(working copy)
@@ -126,6 +126,7 @@
 			'mimeType' => $image->getMimeType(),
 			'srcPath' => $image->getPath(),
 			'dstPath' => $dstPath,
+			'ext' => strtolower(substr( $image->getPath(), strrpos( $image->getPath(), '.' ) + 1 )),
 			'dstUrl' => $dstUrl,
 		);
 
@@ -141,7 +142,7 @@
 		}
 
 		# Determine scaler type
-		$scaler = self::getScalerType( $dstPath );
+		$scaler = self::getScalerType( $scalerParams['ext'], $dstPath );
 		wfDebug( __METHOD__ . ": scaler $scaler\n" );
 
 		if ( $scaler == 'client' ) {
@@ -181,6 +182,9 @@
 			case 'custom':
 				$err = $this->transformCustom( $image, $scalerParams );
 				break;
+			case 'customext':
+				$err = $this->transformCustomExt( $image, $scalerParams );
+				break;
 			case 'imext':
 				$err = $this->transformImageMagickExt( $image, $scalerParams );
 				break;
@@ -213,12 +217,14 @@
 	 *
 	 * @return string client,im,custom,gd
 	 */
-	protected static function getScalerType( $dstPath, $checkDstPath = true ) {
-		global $wgUseImageResize, $wgUseImageMagick, $wgCustomConvertCommand;
+	protected static function getScalerType( $ext, $dstPath, $checkDstPath = true ) {
+		global $wgUseImageResize, $wgUseImageMagick, $wgCustomConvertCommand, $wgCustomConvertCommandsByExtension;
 
 		if ( !$dstPath && $checkDstPath ) {
 			# No output path available, client side scaling only
 			$scaler = 'client';
+		} elseif (array_key_exists($ext, $wgCustomConvertCommandsByExtension)) {
+			$scaler = 'customext';
 		} elseif ( !$wgUseImageResize ) {
 			$scaler = 'client';
 		} elseif ( $wgUseImageMagick ) {
@@ -429,21 +435,19 @@
 	}
 
 	/**
-	 * Transform an image using a custom command
+	 * Invoke an external image transformation command line on a given file, replacing
+	 * %s by the source file path, %d by the destination file path, $w by the width,
+	 * and %h by the height. 
 	 *
-	 * @param $image File File associated with this thumbnail
+	 * @param $cmd string Command line, containing %s, %d, %w, and %h codes.
 	 * @param $params array Array with scaler params
 	 *
 	 * @return MediaTransformError Error object if error occured, false (=no error) otherwise
 	 */
-	protected function transformCustom( $image, $params ) {
-		# Use a custom convert command
-		global $wgCustomConvertCommand;
-
-		# Variables: %s %d %w %h
+	protected function transformCustomExecute( $cmd, $params ) {
 		$src = wfEscapeShellArg( $params['srcPath'] );
 		$dst = wfEscapeShellArg( $params['dstPath'] );
-		$cmd = $wgCustomConvertCommand;
+
 		$cmd = str_replace( '%s', $src, str_replace( '%d', $dst, $cmd ) ); # Filenames
 		$cmd = str_replace( '%h', $params['physicalHeight'],
 			str_replace( '%w', $params['physicalWidth'], $cmd ) ); # Size
@@ -459,8 +463,35 @@
 		}
 		return false; # No error
 	}
+	
+	/**
+	 * Transform an image using a custom command
+	 *
+	 * @param $image File File associated with this thumbnail
+	 * @param $params array Array with scaler params
+	 *
+	 * @return MediaTransformError Error object if error occured, false (=no error) otherwise
+	 */
+	protected function transformCustom( $image, $params ) {
+		# Use a custom convert command
+		global $wgCustomConvertCommand;
+		return $this->transformCustomExecute($wgCustomConvertCommand, $params);
+	}
 
 	/**
+	 * Transform an image using an extension-specific custom command
+	 *
+	 * @param $image File File associated with this thumbnail
+	 * @param $params array Array with scaler params
+	 *
+	 * @return MediaTransformError Error object if error occured, false (=no error) otherwise
+	 */
+	protected function transformCustomExt( $image, $params ) {
+		global $wgCustomConvertCommandsByExtension;
+		return $this->transformCustomExecute($wgCustomConvertCommandsByExtension[$params['ext']], $params);
+	}
+	
+	/**
 	 * Log an error that occured in an external process
 	 *
 	 * @param $retval int
Index: includes/DefaultSettings.php
===================================================================
--- includes/DefaultSettings.php	(revision 95588)
+++ includes/DefaultSettings.php	(working copy)
@@ -637,6 +637,20 @@
 $wgCustomConvertCommand = false;
 
 /**
+ * Use another resizing converter but only for certain
+ * file extensions. Any extensions not included in this
+ * list will be handled by the generic mechanisms above.
+ * This list overrides all other settings for the
+ * included extensions.
+ *
+ * Example for pngscale:
+ * <code>
+ * $wgCustomConvertCommandsByExtension['png'] = "pngscale %s %d %w %h";
+ * </code>
+  */
+$wgCustomConvertCommandsByExtension = array();
+
+/**
  * Some tests and extensions use exiv2 to manipulate the EXIF metadata in some image formats.
  */
 $wgExiv2Command = '/usr/bin/exiv2';