Decode base64 encoded e-mail attachment filename.

According to RFC 2231 the filename of e-mail attachments can be encoded,
e.g. with base64 (for example, the Brother ADS2800W Scanner does this).

Here an example how it looks like:

Content-Disposition: attachment; filename="=?UTF-8?B?QlJXMjg1NjVBOEI5RkQyXzIwMTcwNTIzXzIzMDMzN18wMDAzMDMucGRm?="

Signed-off-by: Cornelius Ludmann <dev@cornelius-ludmann.de>

mayan/apps/sources/models.py

@@ -617,6 +617,12 @@ class EmailBaseModel(IntervalBaseModel):
 
                 if raw_filename:
                     filename = collapse_rfc2231_value(raw_filename)
+
+                    # Decode base64 encoded filename
+                    # https://stackoverflow.com/a/21859258/1364435
+                    if decode_header(filename)[0][1] is not None:
+                        filename = str(decode_header(filename)[0][0]).decode(decode_header(filename)[0][1])
+
                 else:
                     filename = _('attachment-%i') % counter
                     counter += 1