First of, avatars and attachments can be added to the Insight objects, I've attached some screenshots that shows where it can be done.
There is currently no build in support for bulk editing attachments or Avatars.
We do on the other hand have an open API, and there is one question already asked how to upload attachments, which you can read about at https://community.atlassian.com/t5/Marketplace-Apps-questions/Upload-attachment-using-Insight-REST-API/qaq-p/663186
Uploading Avatars is not however possible using the REST API.
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