Pavel Kislinger writes:

This is an imperfection of back-sql. It doesn't exist any way how to say to slapd, which attributes are base64 encoded.

No. base64 has nothing to do with attribute definitions, nor with anything else in LDAP except the LDIF file format. (Some backends make use of the LDIF format internally instead of a more efficient format. What you just said is that back-sql is not one of them.) So:

(...) It doesn't matter, no LDIF file is imported to back-sql.

Exactly.

Looks to me like you need to read up on LDAP and LDIF. E.g. the LDAP wikipedia article. Then, if you still have trouble, please take this to the openldap-software list.