https://addons.mozilla.org/en-US/thunderbird/addon/9271 (Note: Only registered uesr of addons.mozilla.org can download it.)
Go to tools --> Add-ons.
Click on install higligted in red to install new add on.
A new explorer will popup. Path to the extension that you just downloaded. Select it.
After that wait for few seconds to get INSTALL button enabled. Click on the install button highlighted in red and restart Firefox.
Start composing the message and Attach a file to it.
Select the file that you wanted to rename and go to Tools-->Rename Attachment.
When you click on the "Rename Attachment" option a new window will be prompted. Enter the new name for the file in the textbox. click on OK and your file has been renamed.
I hope this is helpful to you. If you have any question or concern or feedback about extension email me on firstname.lastname@example.org