Weird file encoding issue with %CSP.BinaryStream / CSP file upload

Consider the simplest possible CSP file upload/download page - you upload a file, it's saved in the database, and the file is immediately re-download via %CSP.StreamServer:

Class DC.Demo.CSPUpload.Page Extends (%Persistent, %CSP.Page)
{

Property stream As %CSP.BinaryStream [ Required ];

ClassMethod OnPage() As %Status
{
	&html<<form method="post" enctype="multipart/form-data">
		<input type="file" name="file" />
		<input type="submit" />
	</form>>
	quit $$$OK
}

ClassMethod OnPreHTTP() As %Boolean [ ServerOnly = 1 ]
{
	#dim %request As %CSP.Request
	#dim %response As %CSP.Response
	if %request.Method = "POST" && %request.GetMimeData("file") {
		set inst = ..%New()
		set inst.stream = %request.GetMimeData("file")
		$$$ThrowOnError(inst.%Save())
		do inst.stream.SetAttribute("ContentDisposition","attachment; filename="""_inst.stream.GetAttribute("FileName")_"""")
		set %response.Redirect = "%25CSP.StreamServer.cls?STREAMOID="_..Encrypt(inst.stream.GetStreamId())
	}
	quit 1
}

Storage Default
{
<Data name="PageDefaultData">
<Value name="1">
<Value>%%CLASSNAME</Value>
</Value>
<Value name="2">
<Value>stream</Value>
</Value>
</Data>
<DataLocation>^DC.Demo.CSPUpload.PageD</DataLocation>
<DefaultData>PageDefaultData</DefaultData>
<IdLocation>^DC.Demo.CSPUpload.PageD</IdLocation>
<IndexLocation>^DC.Demo.CSPUpload.PageI</IndexLocation>
<StreamLocation>^DC.Demo.CSPUpload.PageS</StreamLocation>
<Type>%Storage.Persistent</Type>
}

}

This works fine for most file types, but for HTML files (at least - maybe other types too), it doesn't work - both on IRIS 2021.2 and on an earlier version I'm running too. The resulting downloaded file has a BOM at the beginning and a NUL every other character.

Has anybody else fought with this before?