<p>Yeah, I'd tracked it down to the Mimetype detection failing, but I was still waiting on getting the raw file to figure out what was triggering it. A byte order mark will definitely do it:</p>
<pre><code>$ bundle exec rails console
Loading development environment (Rails 5.2.3)
irb(main):001:0> MimeMagic.by_magic(StringIO.new("<?xml>")).type
=> "application/xml"
irb(main):002:0> MimeMagic.by_magic(StringIO.new("\xEF\xBB\xBF<?xml>")).type
Traceback (most recent call last):
        1: from (irb):2
NoMethodError (undefined method `type' for nil:NilClass)
irb(main):003:0> 
</code></pre>

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