--- a/python/mozbuild/mozpack/mozjar.py
+++ b/python/mozbuild/mozpack/mozjar.py
@@ -327,24 +327,24 @@ class JarFileReader(object):
         return self._uncompressed_data
 
 
 class JarReader(object):
     '''
     Class with methods to read Jar files. Can open standard jar files as well
     as Mozilla jar files (see further details in the JarWriter documentation).
     '''
-    def __init__(self, file=None, fileobj=None):
+    def __init__(self, file=None, fileobj=None, data=None):
         '''
         Opens the given file as a Jar archive. Use the given file-like object
         if one is given instead of opening the given file name.
         '''
         if fileobj:
             data = fileobj.read()
-        else:
+        elif file:
             data = open(file, 'rb').read()
         self._data = memoryview(data)
         # The End of Central Directory Record has a variable size because of
         # comments it may contain, so scan for it from the end of the file.
         offset = -CDIR_END_SIZE
         while True:
             signature = JarStruct.get_data('uint32', self._data[offset:])[0]
             if signature == JarCdirEnd.MAGIC: