Fix Folder before Open Attachment
On some environments, a substance of a mail folder (mbox file) can become missing while you are reading a message in the folder. In such cases you cannot open any attachment of the reading message due to missing substance file. It may be a bug of Thunderbird itself.
As a workaround, this addon checks existence of the substance file of the folder which contains the message you are reading, and enforces to exist the file before you open any attachment.