[FIX] base: backport of set mimetype on js and css assets

On MS Windows, the mimetypes module relies on the Windows Registry to
guess mimetypes. As a consequence, when generating assets bundle for
javascript files, the mime type of js files may be wrong. In that
particular case, the Odoo user is facing an empty web page.
This commit enforce 'application/javascript' for js assets when they are
saved as ir_attachment in the database.

This commit is a backport of 77ca51f

odoo/addons/base/ir/ir_qweb/assetsbundle.py

@@ -199,12 +199,15 @@ class AssetsBundle(object):
         return self.env['ir.attachment'].sudo().browse(attachment_ids)
 
     def save_attachment(self, type, content, inc=None):
+        assert type in ('js', 'css')
         ira = self.env['ir.attachment']
 
         fname = '%s%s.%s' % (self.name, ('' if inc is None else '.%s' % inc), type)
+        mimetype = 'application/javascript' if type == 'js' else 'text/css'
         values = {
             'name': "/web/content/%s" % type,
             'datas_fname': fname,
+            'mimetype' : mimetype,
             'res_model': 'ir.ui.view',
             'res_id': False,
             'type': 'binary',