[FIX] ir.attachment: simplify xhtml-like mimetypes matching

Simplify/complements revision f6090e69e6,
forcing text/plain for xml-like files.

Ignore forced `mimetype` when downloading attachments, if the model
has its own mimetype or if we can guess it.
`default_mimetype` still available as a fallback.

See also the list of mimetypes supported by common browsers, e.g.:

 - Firefox: CParserContext::SetMimeType in:
    https://hg.mozilla.org/firefox/file/default/parser/htmlparser/CParserContext.cpp

 - Chrome:  kSupportedNonImageTypes constant:
    https://cs.chromium.org/chromium/src/components/mime_util/mime_util.cc?sq=package:chromium&rcl=1467904650&l=89

openerp/addons/base/ir/ir_attachment.py

@@ -212,12 +212,9 @@ class ir_attachment(osv.osv):
 
     def _check_contents(self, cr, uid, values, context=None):
         mimetype = values['mimetype'] = self._compute_mimetype(values)
-        needs_escape = 'htm' in mimetype or '/ht' in mimetype # hta, html, xhtml, etc.
-        if needs_escape and not self.pool['res.users']._is_admin(cr, uid, [uid]):
-            if 'datas' in values:
-                values['datas'] = html_escape(values['datas'].decode('base64')).encode('base64')
-            else:
-                values['mimetype'] = 'text/plain'
+        xml_like = 'ht' in mimetype or 'xml' in mimetype # hta, html, xhtml, etc.
+        if xml_like and not self.pool['res.users']._is_admin(cr, uid, [uid]):
+            values['mimetype'] = 'text/plain'
         return values
 
     def _index(self, cr, uid, bin_data, datas_fname, file_type):

openerp/addons/base/ir/ir_http.py

@@ -295,10 +295,9 @@ class ir_http(osv.AbstractModel):
                 filename = "%s-%s-%s" % (obj._model._name, obj.id, field)
 
         # mimetype
+        mimetype = 'mimetype' in obj and obj.mimetype or False
         if not mimetype:
-            if 'mimetype' in obj and obj.mimetype and obj.mimetype != 'application/octet-stream':
-                mimetype = obj.mimetype
-            elif filename:
+            if filename:
                 mimetype = mimetypes.guess_type(filename)[0]
             if not mimetype and getattr(env[model]._fields[field], 'attachment', False):
                 # for binary fields, fetch the ir_attachement for mimetype check