Fix displaying attached images with wrong Content-Type specified (#5527)

CHANGELOG

@@ -74,6 +74,7 @@ CHANGELOG Roundcube Webmail
 - Fix "Illegal string offset" warning in rcube::log_bug() on PHP 7.1 (#5508)
 - Fix storing "empty" values in rcube_cache/rcube_cache_shared (#5519)
 - Fix missing content check when image resize fails on attachment thumbnail generation (#5485)
+- Fix displaying attached images with wrong Content-Type specified (#5527)
 
 RELEASE 1.2.2
 -------------

program/steps/mail/get.inc

@@ -156,6 +156,10 @@ if (empty($_GET['_thumb']) && $attachment->is_valid()) {
             if ($mimetype == 'application/octet-stream' && strpos($real_mimetype, 'image/') === 0 && $valid_extension) {
                 $mimetype = $real_mimetype;
             }
+            // fix mimetype for images with wrong mimetype
+            else if (strpos($real_mimetype, 'image/') === 0 && strpos($mimetype, 'image/') === 0) {
+                $mimetype = $real_mimetype;
+            }
 
             // "fix" real mimetype the same way the original is before comparison
             $real_mimetype = rcmail_fix_mimetype($real_mimetype);