Fix typo preventing the updating of metadata in MOBI files serverd by the content server

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)
 

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]