Why is Mail attaching replies as .html or .txt documents? (Exchange 2013 EWS)

Late 2014 Retina MacBook Pro (2.5 GHz i7, 16 GB RAM, Nvidia GeForce GT 750M 2 GB)
Mac OS X 10.10.4 Yosemite
Mail.app 8.2 (2102)
Exchange Server 15.0.516.032 (2013)

----

I'm a law student at a university that uses Google Apps, I work for an organization that uses Exchange, and I use Gmail and iCloud for my personal email. As such, Mail.app is a huge part of my workflow. Because of this, using a separate application (e.g., Outlook 2016) just for my work email or migrating all of my accounts to Outlook 2016 doesn't really make sense.

At work, we have a mixed Windows 7/Mac OS X environment, with most users toting work iPhones.

Recently, I've been made aware that when I reply to or forward work email, Mail.app is removing the previous replies from the body of the message and attaching them as either an .html or .txt file instead of appending them to the body of the message. On my MacBook and iPhone (running iOS 8.4), the previous replies are still visible in the body of the message. Since my PC-using colleagues are not used to opening attachments to see previous replies, this leads to a flurry of messages (e.g., "Wait, what was this about?", "Could you please reply to the thread?", etc.) back to me.

If I reply using Outlook 2016 on my MacBook (main work computer), Mail on my iPhone, or the ancient PC at my desk, the replies are appended in the body of the message, as is expected behavior.

What I've Tried:

• Sending messages from Mail.app as plain text. (previous replies were attached as a .txt document instead of .html).
• Copying and pasting previous replies into new messages (with the same subject, so as to preserve threading) (previous replies attached as before, not appended in the message body)
• Updating my settings in OWA to plain text. (no real change, since this would likely only affect messages sent directly through OWA).

Thoughts? I've reached out to our guys here for help, too.