Attachments containing spaces download with + in their filenames in Firefox

Moving the issue from CONF-2487 to here as attachments containing spaces still download with '+'s in their filenames in Firefox.

We still need to address the issue with a work around for Bug 299372:

There is still a bigger issue here when using inline attachments with IE and "Save Link As" in Firefox.

Firefox will use the end of the URL as the filename as it hasn't got the response headers to set the suggested file name. See Mozilla bug #299372 for a long and arduous debate about this issue.