Fix bug in converting multi-page Tiff images into Jpeg (#6824)

When using 'convert' binary we have to use -flatten argument (the same
as we do with thumbnails) otherwise it will produce multiple output files
with -0, -1, etc. suffix. This way we make sure to generate only one image
until we support multi-page Tiff properly.

CHANGELOG

@@ -12,6 +12,7 @@ CHANGELOG Roundcube Webmail
 - Fix bug in rcube_utils::parse_hosts() where %t, %d, %z could return only tld (#6746)
 - Fix bug where Next/Prev button in mail view didn't work with multi-folder search result (#6793)
 - Fix bug where selection of columns on messages list wasn't working
+- Fix bug in converting multi-page Tiff images to Jpeg (#6824)
 
 RELEASE 1.3.9
 -------------

program/lib/Roundcube/rcube_image.php

@@ -324,7 +324,7 @@ class rcube_image
             $p['out']  = $filename;
             $p['type'] = self::$extensions[$type];
 
-            $result = rcube::exec($convert . ' 2>&1 -colorspace sRGB -strip -quality 75 {in} {type}:{out}', $p);
+            $result = rcube::exec($convert . ' 2>&1 -colorspace sRGB -strip -flatten -quality 75 {in} {type}:{out}', $p);
 
             if ($result === '') {
                 chmod($filename, 0600);