mime type macro error with an xslm file
*Created originally on Bitbucket by [ptitjano](https://bitbucket.org/%7Bf8b83496-77ee-48f9-a32a-94993faaeb71%7D/)*
Hi,
The following code produces an error with both version 2.3.5 (from archlinux package) or 2.4 from pip using python 2.7 or 3.5:
```
#!python
wb = load_workbook('Analyse des maitrise.xlsm', keep_vba=True)
ws = wb.get_sheet_by_name('chefs')
ws.cell(column=1, row=1, value=3.14)
wb.save('new_document.xlsm')
```
```
#!python
wb.save('new_document.xlsm')
File "/usr/lib/python2.7/site-packages/openpyxl/workbook/workbook.py", line 339, in save
save_workbook(self, filename)
File "/usr/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 268, in save_workbook
writer.save(filename)
File "/usr/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 250, in save
self.write_data()
File "/usr/lib/python2.7/site-packages/openpyxl/writer/excel.py", line 98, in write_data
self.manifest._write(archive, self.workbook)
File "/usr/lib/python2.7/site-packages/openpyxl/packaging/manifest.py", line 167, in _write
archive.writestr(self.path, tostring(self.to_tree()))
File "/usr/lib/python2.7/site-packages/openpyxl/packaging/manifest.py", line 124, in to_tree
for ext, mime in self.extensions:
File "/usr/lib/python2.7/site-packages/openpyxl/packaging/manifest.py", line 116, in extensions
return [(ext[1:], mimetypes.types_map[ext]) for ext in sorted(exts)]
KeyError: ''
```
*Attachments:* [Analyse_des_maitrise.xlsm](/uploads/6ba5b6a5cc25f223cd54c21cf8727aa6/Analyse_des_maitrise.xlsm)
issue