FYI - I have raised a support case regarding this question, and will share what I learn.
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Reviewing through your description, this appears to be an issue being tracked as CLOUD-4493, where DXF files are reported as "too large" when uploading them into a Jive instance. Reviewing through the bug's notes, one potential workaround for this behavior is to zip up the file prior to uploading it into your instance. Additionally, this behavior can be confirmed and tracked in your support case.