Can't import attachment by CSV on a JIRA instance with a https URL of another JIRA instance

[GRT ENEDIS]

We are trying to export issues from a JIRA instance in version 6.1.2 to another JIRA instance in version 7.2.6.
The issues we want to export are selected and formated with a SQL query, form the 6.1.2 instance, then saved as a csv file to use the JIRA csv importer of the 7.2.6 instance.
We are adding the attachments within the SQL query, so we have something like : https://our_jira_url/secure/attachment/attachment_id/attachment_name , in the column "Attachment" of our csv file.

When the csv import is launched, I get no mistakes, the import goes well.
I have an issue that is created, with an attached file which has the correct name.
But when I open it, I don't have the content of the original attachment but another

I have tried to had "username:password@" in the url: https://username:password@our_jira_url/secure/attachment/attachment_id/attachment_name  but I have the same results.

When we try the url below in a browser from the jira instance 7.2.6 or from another place, it works, we get the file.

[Ash]

@GRT ENEDISwhy don't you sync Jira 6.1.2 attachment folder with Jira 7.2.6 instance attachment folder.

I prefer upgrading Jira 6.1.2 to 7.2.6 then take a xml and import in new Jira 7.2.6 instance. There are plugins to do this as well.

Take a look at Project configurator.