diff --git a/src/calibre/ebooks/conversion/plugins/epub_input.py b/src/calibre/ebooks/conversion/plugins/epub_input.py
index c5742f2c7d..b6f857ef1f 100644
--- a/src/calibre/ebooks/conversion/plugins/epub_input.py
+++ b/src/calibre/ebooks/conversion/plugins/epub_input.py
@@ -294,11 +294,22 @@ class EPUBInput(InputFormatPlugin):
         not_for_spine = set()
         for y in opf.itermanifest():
             id_ = y.get('id', None)
-            if id_ and y.get('media-type', None) in {
-                    'application/vnd.adobe-page-template+xml', 'application/vnd.adobe.page-template+xml',
-                    'application/adobe-page-template+xml', 'application/adobe.page-template+xml',
-                    'application/text'}:
-                not_for_spine.add(id_)
+            if id_:
+                mt = y.get('media-type', None)
+                if mt in {
+                        'application/vnd.adobe-page-template+xml',
+                        'application/vnd.adobe.page-template+xml',
+                        'application/adobe-page-template+xml',
+                        'application/adobe.page-template+xml',
+                        'application/text'
+                }:
+                    not_for_spine.add(id_)
+                ext = y.get('href', '').rpartition('.')[-1].lower()
+                if mt == 'text/plain' and ext in {'otf', 'ttf'}:
+                    # some epub authoring software sets font mime types to
+                    # text/plain
+                    not_for_spine.add(id_)
+                    y.set('media-type', 'application/font')
 
         seen = set()
         for x in list(opf.iterspine()):