Need migrationhelp for 1.3.6.1.4.1.1466.115.121.1.5 because of bug in slapcat

[Frank Offermanns <Frank.Offermanns@caseris.de>]

Hello,

in our custom schema we have used several
attributes with the syntax: 
1.3.6.1.4.1.1466.115.121.1.5
attributetype ( myAttributeType:106
NAME ( 'casFileContent' )
        DESC
'Contains the file'
        SYNTAX 1.3.6.1.4.1.1466.115.121.1.5
        SINGLE-VALUE
)

We defined this schema before this syntax
was dropped.

Everything worked for a long time. Now
with OpenLDAP Version 2.4.39 and newer a slapcat does  no longer export
this attributes as MimeContent, but as plain text. So the ldif can't be
used for importing. So I started a research why and found that this syntax
was dropped. But for downward compatibility shouldn't it still export this
syntax as mime content, shouldt it? 
This would be important, since in inetorgperson.schema
the 2 attributes userSMIMECertificate and userPKCS12 still use it.

What syntax should I use for files stored
in LDAP?
And even more important, how can I change
the syntax of a attribute. Is there a migration strategy to change the
syntax of a attribute?

Regards,
Frank