Office OpenXML files files (.docx, .xlsx) get corrupted

[tiangolo]

Office OpenXML files in attachments seem to be corrupted.

Steps to reproduce:

• Send an encrypted email with an Office file attached using the new version (OpenXML): .docx or .xlsx.

• On the other end, after receiving the message, download the attached MS Office file.

• The downloaded file is unnamed, so the browser uses just "download" as the name of the file (that's the first issue).

• After renaming that to the original file name with the original extension, and trying to open it with MS Office (Word, Excel, etc). It throws an error telling that the file is corrupt and is unable to read or recover it.