Safari changes file extension when downloading file

Using Snow Leopard and Safari to access my email online, attachments which are .wmv files are having a .flv extension appended to the file name when they are downloaded.

Thus the file appeared in the Downloads folder as english.wmv.flv.

Besides happening on my iMac, it happens on my MacBook and on a MacBook Pro at a local Apple store.

I can download the attachment correctly using Firefox, Chrome, or the Mail app on my iMac or MacBook; using Safari on my older iMacs running OS X 10.2 and OS X 10.5; and on a Windows computer using Outlook or any browser (including Safari). In short, the .flv extension is only added using the current Mac OS with the current version of Safari.

This doesn't happen on every .wmv file attached to an email. I have taken an email attachment that is problematic with Safari on the Mac, attached it to a new email using a Windows computer, and not have a problem with the new message on the Mac.

But, on those messages with attachments on which it does happen, it happens happens every time. It is reproducible.

In any case, I would like to know why Safari would feel it necessary, in any case, to append an extension to a file that is being downloaded. Mail on OS X doesn't. Chrome on OS X doesn't. Firefox on OS X doesn't. Outlook on Windows doesn't. Four different browsers on Windows don't (including Safari on Windows).

Just what is this new feature in Safari that assumes (incorrectly) that a file being downloaded has the wrong file extension and that it needs to be changed automatically? Can I disable this?