tl;dr: Be as clear as you can be, to avoid confusion for re-users.
When you take a file from one project, and inject it in to another, you are creating a combined work and/or a derivative work (depending on the amount of coupling between them).
This is not a problem. The general gist of things is answered well at Combining code written under different licenses (Eiffel Forum License, MIT and Apache), what are my options?, especialy Abhi Beckerts answer.
What you are looking to do is to make it abundantly clear to people looking at your project what the copyright status is. In your case specifically I would include a notice file which describes the copyright situation of the entire thing. It would look something like
Copyright 2015 < you >
Copyright < year of copyright BSD file > < author BSD file >
This project is licensed under the MIT license, with exception of < that file > which is licensed under the 3 clause BSD license
Then I'd include a copyright notice header for the BSD licensed file, with the copyright holder and that it's under the BSD license.
For each license, include a text file with the license text in your project (but name neither of them just plain License, as that could make people think that's the license of everything)
Now you have
- A notice file that describes the situation of the entire project (MIT + 1 file BSD)
- The license text of each license
- A copyright header in each file that describes the copyright situation for that specific file.
Someone coming to your project now has very little chance to be confused about the copyright and licensing of each file and project as a whole.
