diff --git a/src/calibre/ebooks/metadata/mobi.py b/src/calibre/ebooks/metadata/mobi.py
index c9c7d80902..cf8fdbd7e8 100644
--- a/src/calibre/ebooks/metadata/mobi.py
+++ b/src/calibre/ebooks/metadata/mobi.py
@@ -78,7 +78,7 @@ class StreamSlicer(object):
         stream = self._stream
         base = self.start
         stream.seek(base)
-        self._stream.truncate(base)
+        stream.truncate()
         for block in data_blocks:
             stream.write(block)
 
diff --git a/src/calibre/library/server/content.py b/src/calibre/library/server/content.py
index bb6fe1c454..983dae8aae 100644
--- a/src/calibre/library/server/content.py
+++ b/src/calibre/library/server/content.py
@@ -218,7 +218,7 @@ class ContentServer(object):
         if format in ('MOBI', 'EPUB'):
             # Write the updated file
             from calibre.ebooks.metadata.meta import set_metadata
-            set_metadata(fmt, newmi, 'epub')
+            set_metadata(fmt, newmi, format.lower())
             fmt.seek(0)
 
         mt = guess_type('dummy.'+format.lower())[0]