[exim] Problem with filter on Base64-encoded subject field

[Kirill Sluchanko via Exim-users]

Hi,
I have to filter out some tags from outgoing email subject field. I use the 
following filter:
 
    if $h_subject: matches \N\[\NTAG1|TAG2|TAG3\N\]\N
    then
        headers add "tmpSubject: 
${sg{$bh_subject:}{\\\\[(TAG1|TAG2|TAG3)\\\\]\\\\ }{}}"
        headers remove "Subject"
        headers add "Subject: $h_tmpSubject"
        headers remove "tmpSubject"
    endif
 
and it works fine while users send emails with email clients and subject is 
ISO-8859-1 plain text. But some users do it with Google Mail’s web interface. 
Then outgoing email subject is in UTF-8 and encoded with Base64. Based on 
«String expansion» section of documentation, I have tried to add second stage 
to the filter with the same content, but prepended with 
 
    headers charset "UTF-8"
 
- no luck.
Then I’ve tried to determine the problem. I’ll bypass some intermediate steps, 
but I have ended with following in filter:
 
    headers charset "UTF-8" 
    headers add "tmpB64decodedSubject: $bh_subject" 
    headers add "hSubject: $h_subject"
 
In my understanding of «String expansion» this both should result with 
additional headers, decoded from Base64 — however, I’m getting three identical 
headers:
 
    Subject: 
=?UTF-8?B?UmU6IFtFWFRFUk5BTF0gUmU6IFJlOiBSZTog0JzQsNGA0YjRgNGD0YLQuNC30LDRhtC4?=
 я почты
    tmpB64decodedSubject: 
=?UTF-8?B?UmU6IFtFWFRFUk5BTF0gUmU6IFJlOiBSZTog0JzQsNGA0YjRgNGD0YLQuNC30LDRhtC4?=
 я почты
    hSubject: 
=?UTF-8?B?UmU6IFtFWFRFUk5BTF0gUmU6IFJlOiBSZTog0JzQsNGA0YjRgNGD0YLQuNC30LDRhtC4?=
 я почты   
 
Once again, according to my understanding of «String expansion» this means that 
Exim cannot decode the original header for some reason. I have tried to decode 
« UmU6IFtFWFRFUk5BTF0gUmU6IFJlOiBSZTog0JzQsNGA0YjRgNGD0YLQuNC30LDRhtC4» 
manually on the same server with 
 
    echo   UmU6IFtFWFRFUk5BTF0gUmU6IFJlOiBSZTog0JzQsNGA0YjRgNGD0YLQuNC30LDRhtC4 
| base64 -d
 
and it works fine:
 
    Re: [EXTERNAL] Re: Re: Re: Маршрутизаци
 
(tag [EXTERNAL] is one of the actual tags in use).
What am I doing wrong in filters? Or how can I do further diagnostic to 
determine why Exim cannot decode the headers? Does multiline in subject header 
influences the result? 
 
Best regards,
Kirill Sluchanko
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