Some email attachments fail on getting filename

[ElliotJH]

Sometimes

@property
    def filename(self):
        return self.mfs['PidTagAttachLongFilename']

Fails because PidTagAttachLongFIlename isn't valid.

[ElliotJH]

This is when an email is attached to an email, generally on an internal forwarded emails

[PierreSelim]

@ElliotJH encounter the same problem, not sure where the filename of the attachment is store for this, the MAPI reference is quite a mess to be honnest :)

I believe we need to read it from the subject of the attached email:

• Read the attachment
• Open it as a new msg
• Retrieve the subject from the msg and add '.msg' to the attached file for it's filename