There are two files included in the CSS folder with .map file extensions. They are:
bootstrap-theme.css.map
bootstrap.css.map
They appear to be minified files but I don't know what they are for.
From Working with CSS preprocessors in Chrome DevTools:
Many developers generate CSS style sheets using a CSS preprocessor, such as Sass, Less, or Stylus. Because the CSS files are generated, editing the CSS files directly is not as helpful.
For preprocessors that support CSS source maps, DevTools lets you live-edit your preprocessor source files in the Sources panel, and view the results without having to leave DevTools or refresh the page. When you inspect an element whose styles are provided by a generated CSS file, the Elements panel displays a link to the original source file, not the generated .css file.