MediaTypePlugin.fileMatcher should take the URI

... as opposed to the os-dependent file system path.

lib/uri-schemes/file-scheme-plugin.js

@@ -22,7 +22,7 @@ const retrieve = async (uri, baseUri) => {
     responseUri = pathToFileURL(await readlink(filePath)).toString();
   }
 
-  const contentType = await getFileMediaType(filePath);
+  const contentType = await getFileMediaType(responseUri);
   const stream = createReadStream(filePath);
   const response = new Response(stream, {
     headers: { "Content-Type": contentType }