Fix broken long filenames when using imap4d server - workaround server bug (#6048)

CHANGELOG

@@ -51,6 +51,7 @@ CHANGELOG Roundcube Webmail
 - Fix touch event issue on messages list in IE/Edge (#5781)
 - Fix so links over images are not removed in plain text signatures converted from HTML (#4473)
 - Fix various issues when downloading files with names containing non-ascii chars, use RFC 2231 (#5772)
+- Fix broken long filenames when using imap4d server - workaround server bug (#6048)
 
 RELEASE 1.3.3
 -------------

program/lib/Roundcube/rcube_mime.php

@@ -217,9 +217,21 @@ class rcube_mime
 
                 // Decode and join encoded-word's chunks
                 if ($encoding == 'B' || $encoding == 'b') {
-                    // base64 must be decoded a segment at a time
-                    for ($i=0; $i<$count; $i++)
-                        $text .= base64_decode($tmp[$i]);
+                    $rest  = '';
+                    // base64 must be decoded a segment at a time.
+                    // However, there are broken implementations that continue
+                    // in the following word, we'll handle that (#6048)
+                    for ($i=0; $i<$count; $i++) {
+                        $chunk  = $rest . $tmp[$i];
+                        $length = strlen($chunk);
+                        if ($length % 4) {
+                            $length = floor($length / 4) * 4;
+                            $rest   = substr($chunk, $length);
+                            $chunk  = substr($chunk, 0, $length);
+                        }
+
+                        $text .= base64_decode($chunk);
+                    }
                 }
                 else { //if ($encoding == 'Q' || $encoding == 'q') {
                     // quoted printable can be combined and processed at once