Untitled attachments in Apple Mail

When I send a valid email attachment from my Apple Mail to colleagues using Windows PCs, they sometimes receive an additional file, typically titled either 'Untitled Attachment 000XX.txt' or 'Untitled Attachment 000XX.htm', where XX is a random two-digit number following 3 zeros. These files appear to be empty when opened with TextEdit or Safari, but on at least one occasion, my colleague reports that it contained the full text of another, unrelated email between my colleague and her colleague, from weeks earlier. Any ideas please?

Mac OS X Yosemite 10.10.1 with Mail 8.1