When content::item::upload_file is used, the name is rewritten without an extension. This often confuses mail clients. However, cr_revisions.title retains the original filename.

Any complaints if I change the attachment filename to use cr_revisions.title instead of cr_items.name?

Good point regarding a null title. I am using 5.6.

The name is sanitized by util_text_to_url in upload_file.

I would suggest changing the regexp to allow for a period:

# substitute all non-word characters
regsub -all {([^a-z0-9\.])+} $text $replacement text

This sets the name with a proper extension unless there are name conflicts, in which case file.pdf becomes file.pdf_2.

It's better than modifying the extension IMO.