Sending gzip compressed version or any other file (like MS Word documents or images) results in receiving proper attachment so this must be file-specific. Sending first 50 KB results in receiving plaintext base64. It was tested with several mailboxes, including Yandex and GMail ones so this must be claws-mail issue.
Created attachment 1967 [details] First 50 KB of problematic file
Changes related to this bug have been committed. Please check latest Git and update the bug accordingly. You can also get the patch from: http://git.claws-mail.org/ ++ ChangeLog 2019-03-24 14:40:03.547669893 +0100 http://git.claws-mail.org/?p=claws.git;a=commitdiff;h=0c0a3c773b657bb80f4af3b7dbfd9646766bb526 Author: Paul <paul@claws-mail.org> Date: Sun Mar 24 13:38:58 2019 +0000 recognise model/* mime type fixes bug 4188, 'STL file is sent not as an attachment but as its base64 representation in plaintext'
FYI: The fix won't work retrospectively, but will work going forward. That is, old messages remain broken, but new messages will have the correct mime type.