1 files changed, 11 insertions, 1 deletions
diff --git a/mammoth/docx/files.py b/mammoth/docx/files.py
index d919de9..7fe3a2c 100644
--- a/mammoth/docx/files.py
+++ b/mammoth/docx/files.py
@@ -14,12 +14,22 @@ class Files(object):
     def __init__(self, base):
         self._base = base
     
+    def verify(self, uri):
+        with self.open(uri):
+            pass
+    
     def open(self, uri):
         if _is_absolute(uri):
             return contextlib.closing(urlopen(uri))
-        else:
+        elif self._base is not None:
             return open(os.path.join(self._base, uri), "rb")
+        else:
+            raise InvalidFileReferenceError("could not find external image '{0}', fileobj has no name".format(uri))
 
 
 def _is_absolute(url):
     return urlparse(url).scheme != ""
+
+
+class InvalidFileReferenceError(ValueError):
+    pass