Hi!
Element <submission_file> has attribute file_id = " " and when it corresponds to child <file id=" " import works.
I didn’t find xml example for import old issues to ojs 3.4, so try-checked different variants, and succeeded with this:
<?xml version="1.0" encoding="utf-8"?>
<issues xmlns="http://pkp.sfu.ca" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<issue xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" published="1" current="0" access_status="1">
<id type="public" advice="update">0000-0000-5-3</id>
<description locale="ru">Журнал том 5 № 3 2013</description>
<description locale="en">Journal vol. 5 No. 3 2013</description>
<issue_identification>
<volume>5</volume>
<number>3</number>
<year>2013</year>
<title locale="ru">Журнал том 5 № 3 2013</title>
<title locale="en">Journal vol. 5 No. 3 2013</title>
</issue_identification>
<date_published>2013-09-20</date_published>
<last_modified>2013-09-20</last_modified>
<sections>
<section seq="2" abstract_word_count="0">
<abbrev locale="en">THEORY</abbrev>
<abbrev locale="ru">THEORY</abbrev>
<title locale="en">THEORY</title>
<title locale="ru">ТЕОРИЯ</title>
</section>
<section seq="3" abstract_word_count="0">
<abbrev locale="en">NATURE</abbrev>
<abbrev locale="ru">NATURE</abbrev>
<title locale="en">NATURE</title>
<title locale="ru">ПРИРОДА</title>
</section>
</sections>
<articles xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<article xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" locale="ru" date_submitted="2013-09-20" status="3" submission_progress="0" stage="production" current_publication_id="1">
<submission_file xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="1" stage="proof" created_at="2013-09-20" updated_at="2013-09-20" viewable="true" genre="Article Text" uploader="oleg" file_id="1" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<name locale="ru">NameForReader(Downloader)</name>
<file id="1" filesize="8297738" extension="pdf">
<href src="files/file.pdf"/>
</file>
</submission_file>
<publication xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1" status="3" section_ref="THEORY" seq="0" date_published="2013-09-20" access_status="1" xsi:schemaLocation="http://pkp.sfu.ca native.xsd" primary_contact_id="0">
<id type="internal" advice="ignore">1</id>
<title locale="ru">Название статьи</title>
<title locale="en">Title Of Article</title>
<abstract locale="ru">Аннотация</abstract>
<abstract locale="en">Abstract</abstract>
<licenseUrl>https://creativecommons.org/licenses/by-nc-nd/4.0</licenseUrl>
<copyrightHolder locale="en">Journal</copyrightHolder>
<copyrightHolder locale="ru">Журнал</copyrightHolder>
<copyrightYear>2013</copyrightYear>
<keywords locale="ru">
<keyword>почва</keyword>
<keyword>мегаполис</keyword>
</keywords>
<keywords locale="en">
<keyword>soil</keyword>
<keyword>megalopolis</keyword>
</keywords>
<authors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<author id="0" seq="1" user_group_ref="Author" primary_contact="true">
<givenname locale="ru">А.А.</givenname>
<givenname locale="en">A.A.</givenname>
<familyname locale="ru">Фамилия</familyname>
<familyname locale="en">FamilyName</familyname>
<affiliation locale="ru">Университет</affiliation>
<affiliation locale="en">University</affiliation>
<country>RU</country>
<email>noreply@univ.edu</email>
</author>
<author id="0" seq="2" user_group_ref="Author" primary_contact="false">
<givenname locale="ru">Б.Б.</givenname>
<givenname locale="en">B.B.</givenname>
<familyname locale="ru">Фамилия</familyname>
<familyname locale="en">FamilyName</familyname>
<affiliation locale="ru">Аффилиация</affiliation>
<affiliation locale="en">Affiliation</affiliation>
<country>RU</country>
<email>noreply@univ.edu</email>
</author>
</authors>
<article_galley xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" locale="ru" url_path="" approved="true" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<name locale="ru">PDF</name>
<seq>0</seq>
<submission_file_ref id="1"/>
</article_galley>
<citations>
<citation>Литература</citation>
<citation>Статья</citation>
</citations>
</publication>
</article>
<article xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" locale="ru" date_submitted="2013-09-20" status="3" submission_progress="0" stage="production" current_publication_id="1">
<submission_file xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" id="1" stage="proof" created_at="2013-09-20" updated_at="2013-09-20" viewable="true" genre="Article Text" uploader="oleg" file_id="2" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<name locale="ru">OtherNameForReader.pdf</name>
<file id="2" filesize="807942" extension="pdf">
<href src="files/otherfile.pdf"/>
</file>
</submission_file>
<publication xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" version="1" status="3" section_ref="NATURE" seq="0" date_published="2013-09-20" access_status="1" xsi:schemaLocation="http://pkp.sfu.ca native.xsd" primary_contact_id="0">
<id type="internal" advice="ignore">1</id>
<title locale="ru">Название следующей статьи</title>
<title locale="en">Next Article Title</title>
<abstract locale="ru">Аннотация</abstract>
<abstract locale="en">Abstract</abstract>
<licenseUrl>https://creativecommons.org/licenses/by-nc-nd/4.0</licenseUrl>
<copyrightHolder locale="en">Journal</copyrightHolder>
<copyrightHolder locale="ru">Журнал</copyrightHolder>
<copyrightYear>2013</copyrightYear>
<keywords locale="ru">
<keyword>система</keyword>
<keyword>модель</keyword>
</keywords>
<keywords locale="en">
<keyword>system</keyword>
<keyword>model</keyword>
</keywords>
<authors xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<author id="0" seq="1" user_group_ref="Author" primary_contact="true">
<givenname locale="ru">А.А.</givenname>
<givenname locale="en">A.A.</givenname>
<familyname locale="ru">Фамилия</familyname>
<familyname locale="en">FamilyName</familyname>
<affiliation locale="ru">Аффилиация</affiliation>
<affiliation locale="en">Affiliation</affiliation>
<country>RU</country>
<email>noreply@univ.edu</email>
</author>
<author id="0" seq="2" user_group_ref="Author" primary_contact="false">
<givenname locale="ru">Б.Б.</givenname>
<givenname locale="en">B.B.</givenname>
<familyname locale="ru">Фамилия</familyname>
<familyname locale="en">FamilyName</familyname>
<affiliation locale="ru">Университет</affiliation>
<affiliation locale="en">University</affiliation>
<country>RU</country>
<email>noreply@univ.edu</email>
</author>
</authors>
<article_galley xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" locale="ru" url_path="" approved="true" xsi:schemaLocation="http://pkp.sfu.ca native.xsd">
<name locale="ru">PDF</name>
<seq>0</seq>
<submission_file_ref id="1"/>
</article_galley>
<citations>
<citation>Литература</citation>
<citation>Статья</citation>
</citations>
</publication>
</article>
</articles>
</issue>
</issues>
With 2 caveats:
- import from local file href=" " works in cli mode, not in interface
- table ‘event_log’ index constraint to ‘user_id’ table must be deleted for time of import, couldn’t understand for bug of what.
Hope it helps!