Provide better support for file-like objects with replace-media

docxtpl/__init__.py

@@ -456,8 +456,9 @@ class DocxTemplate(object):
 
             if hasattr(docx_file, 'read'):
                 tmp_file = io.BytesIO()
-                Document(docx_file).save(tmp_file)
+                DocxTemplate(docx_file).save(tmp_file)
                 tmp_file.seek(0)
+                docx_file.seek(0)
 
             else:
                 tmp_file = '%s_docxtpl_before_replace_medias' % docx_file

tests/header_footer_image_file_obj.py

@@ -32,4 +32,6 @@ tpl.render(context)
 file_obj = io.BytesIO()
 tpl.save(file_obj)
 file_obj.seek(0)
-DocxTemplate(file_obj).save(DEST_FILE2)
+with open(DEST_FILE2, 'wb') as f:
+    f.write(file_obj.read())
+