I can confirm this behavior with small PDF files. Not sure about other types. Just spent a frustrating few hours with Apple support on this issue. Too bad they don't know their own product. I went through the usual routine of reboot, delete mail account, reset settings, etc. before they sent me on to a senior tech. He didn't know what was up either, but after a long time he confirmed that this is the new functionality.
The support rep seems to think this was a change in 6.0.1, or maybe even 6. If the file is small, not too complex, and the mail reader app can decode it, then the file displays its contents directly in the email. The little download box we all got used to no longer exists for these small files, nor does the secondary screen where the file would open and there was an option to send the file off to different apps such as GoodReader. How one is to send small files to another app is a mystery to me. The rep indicated you just hold your finger down on the file display inside the email and a "send" menu should come up, but it doesn't work that way for me.
Apple seems to specialize in removing functionality from iOS products these days. The maps app, the contacts search that now also searches the notes field returning way too many names, and now this blooper. My next tablet and phone won't be from Apple, that is clear.