Colleague is using TNEF otherwise you would not have a winmail file. This is a microsoft specific issue and there are a few different ways of switching it off. So maybe it has not been switched off in the manner your colleague thinks. All this info is available via an easy Google search for outlook tnef winmail

Links are difficult to create at the moment so I'm copy pasting directly from the microsft webpage.

Both Microsoft Outlook and the Microsoft Exchange Client sometimes use a special method to package information for sending messages across the Internet. This method is technically known as Transport Neutral Encapsulation Format known as TNEF.

A TNEF encoded message contains a plain text version of the message, and a binary attachment that packages various other parts of the original message. In most cases, the binary attachment is named Winmail - which contains a load of info that only microsoft product can by default read. If you are sending messages that have file attachments to a recipient who does not use Outlook or the Exchange Client, we recommend that you manually choose to use an email format which does not require TNEF (such as HTML or plain text). If a message is sent without TNEF, the recipient can view and save the attachments as expected.

How to control TNEF in messages

You can control TNEF by using one of three methods:

• Global - If you change your default email format to plain text or HTML, it helps make sure that TNEF is not sent unless an Outlook feature needs it.
• Per Recipient - You can specify in the recipient's email address not to send TNEF so that a recipient always receives plain text versions of the message.
• Per Message - When you compose a new message, or reply to a received message.

Method 1. How to make a global change for TNEF

For Microsoft Outlook 2010 and later versions:

To turn off TNEF, follow these steps:

• Click the File tab, click Options, and then click Mail.
• In the Compose in this message format list, click Plain Text or HTML, and then click OK.

To send messages in TNEF, follow these steps:

• Click the File tab, click Options, and then click Mail.

Method 2 How to make a Per Recipient Change for TNEF

For Outlook 2010:

In the Contacts Folder

To turn off TNEF, follow these steps:

• Open the recipient's record in the Contacts folder.
• Double click the recipient's email address.
• The Contact Card appears. Click View more options for interacting with this person, and then click Outlook properties.
• Select Send Plain Text only in the Internet Format list.

To send in TNEF, follow these steps: 

• Open the recipient's record in the Contacts folder.
• Double click the recipient's email address.
• The Contact Card appears. Click View more options for interacting with this person, and then click Outlook properties.
• Click Send Using Outlook Rich Text Format in the Internet Format box.

Method 3 How to make a per-message change for TNEF

For Outlook 2010 and later versions:

To turn off TNEF, follow these steps:

• Open a new mail message, or click Reply on a received message.
• On the Format Text tab, click HTML or Plain Text.

To turn on TNEF, follow these steps:

• Open a new mail message, or click Reply on a received message.
• On the Format Text tab, click Rich Text.

Sorry ignore last post as the link does not work correctly.

Install the Lookout add-on.

https://support.mozilla.org/en-US/kb/installing-addon-thunderbird

Good to hear that addon has been updated to work on current versions.