diff --git a/mitogen/service.py b/mitogen/service.py
index db042041..0bafc899 100644
--- a/mitogen/service.py
+++ b/mitogen/service.py
@@ -754,8 +754,8 @@ class FileService(Service):
 
     def __init__(self, router):
         super(FileService, self).__init__(router)
-        #: Mapping of registered path -> file size.
-        self._metadata_by_path = {}
+        #: Set of registered paths.
+        self._paths = set()
         #: Mapping of Stream->FileStreamState.
         self._state_by_stream = {}
 
@@ -777,15 +777,16 @@ class FileService(Service):
         :param str path:
             File path.
         """
-        if path in self._metadata_by_path:
-            return
+        if path not in self._paths:
+            LOG.debug('%r: registering %r', self, path)
+            self._paths.add(path)
 
+    def _generate_stat(self, path):
         st = os.stat(path)
         if not stat.S_ISREG(st.st_mode):
             raise IOError('%r is not a regular file.' % (path,))
 
-        LOG.debug('%r: registering %r', self, path)
-        self._metadata_by_path[path] = {
+        return {
             'size': st.st_size,
             'mode': st.st_mode,
             'owner': self._name_or_none(pwd.getpwuid, 0, 'pw_name'),
@@ -869,25 +870,25 @@ class FileService(Service):
         :raises Error:
             Unregistered path, or Sender did not match requestee context.
         """
-        if path not in self._metadata_by_path:
+        if path not in self._paths:
             raise Error(self.unregistered_msg)
         if msg.src_id != sender.context.context_id:
             raise Error(self.context_mismatch_msg)
 
         LOG.debug('Serving %r', path)
-        try:
-            fp = open(path, 'rb', self.IO_SIZE)
-        except IOError:
-            msg.reply(mitogen.core.CallError(
-                sys.exc_info()[1]
-            ))
-            return
 
         # Response must arrive first so requestee can begin receive loop,
         # otherwise first ack won't arrive until all pending chunks were
         # delivered. In that case max BDP would always be 128KiB, aka. max
         # ~10Mbit/sec over a 100ms link.
-        msg.reply(self._metadata_by_path[path])
+        try:
+            fp = open(path, 'rb', self.IO_SIZE)
+            msg.reply(self._generate_stat(path))
+        except IOError:
+            msg.reply(mitogen.core.CallError(
+                sys.exc_info()[1]
+            ))
+            return
 
         stream = self.router.stream_by_id(sender.context.context_id)
         state = self._state_by_stream.setdefault(stream, FileStreamState())