Finish the references implementation for the extraction module and update roadmap documentation

docs/troubleshooting.md

@@ -113,6 +113,7 @@ Symptoms include:
 - `--without-reference must use slot or lane.slot`
 - `read "...": no such file`
 - `must be UTF-8 text`
+- `media type "..." is not accepted`
 - `is ... bytes, limit ...`
 
 Fix:
@@ -127,6 +128,9 @@ Fix:
   directory.
 - Ensure the file is readable UTF-8 text and within any byte limit declared by
   the extractor.
+- If the extractor narrows accepted media types, use a file extension that
+  infers an accepted type such as `text/markdown` or `application/json`.
+  Unknown extensions infer `application/octet-stream`.
 - If diagnostics are retained, inspect `resolved-pipeline.json`,
   `resolved-references.json`, and `error.log`.