iXML and media types in XProc

When I feed a iXML grammar with a `.ixml` extension into an XProc pipeline using a port, I often encounter `Input with content type "application/octet-stream" not allowed on port "grammar"`. Is it possible for this (or another standard extension) extension to be recognized as text/plain as `.txt` is? Perhaps I'm not understanding media type detection for filesystem uris.

```xml
<p:declare-step xmlns:p=http://www.w3.org/ns/xproc name="pipeline" version="3.0">
    <p:input port="grammar" primary="true" />
    <p:input port="source" />
    <p:output port="result" primary="true" />
    <p:invisible-xml>
        <p:with-input port="grammar" pipe="grammar@pipeline"/>
        <p:with-input port="source" pipe="source@pipeline"/>
    </p:invisible-xml>
</p:declare-step>
```

John Dziurłaj /d͡ʑurwaj/