diff --git a/src/calibre/ebooks/conversion/plugins/mobi_input.py b/src/calibre/ebooks/conversion/plugins/mobi_input.py
index 55e3c7f0d3..27729ff4f0 100644
--- a/src/calibre/ebooks/conversion/plugins/mobi_input.py
+++ b/src/calibre/ebooks/conversion/plugins/mobi_input.py
@@ -17,6 +17,7 @@ class MOBIInput(InputFormatPlugin):
     def convert(self, stream, options, file_ext, log,
                 accelerators):
         self.is_kf8 = False
+        self.mobi_is_joint = False
 
         from calibre.ebooks.mobi.reader.mobi6 import MobiReader
         from lxml import html
@@ -35,6 +36,8 @@ class MOBIInput(InputFormatPlugin):
 
         if mr.kf8_type is not None:
             log('Found KF8 MOBI of type %r'%mr.kf8_type)
+            if mr.kf8_type == 'joint':
+                self.mobi_is_joint = True
             from calibre.ebooks.mobi.reader.mobi8 import Mobi8Reader
             mr = Mobi8Reader(mr, log)
             opf = os.path.abspath(mr())
diff --git a/src/calibre/ebooks/oeb/iterator/book.py b/src/calibre/ebooks/oeb/iterator/book.py
index d3cedce688..8868e7cbb1 100644
--- a/src/calibre/ebooks/oeb/iterator/book.py
+++ b/src/calibre/ebooks/oeb/iterator/book.py
@@ -113,7 +113,8 @@ class EbookIterator(BookmarksMixin):
 
         self.book_format = os.path.splitext(self.pathtoebook)[1][1:].upper()
         if getattr(plumber.input_plugin, 'is_kf8', False):
-            self.book_format = 'KF8'
+            fs = ':joint' if getattr(plumber.input_plugin, 'mobi_is_joint', False) else ''
+            self.book_format = 'KF8' + fs
 
         self.opf = getattr(plumber.input_plugin, 'optimize_opf_parsing', None)
         if self.opf is None: