You can put any file type into your /content directories, but Hugo uses the markup front matter value if set or the file extension (see Markup identifiers in the table below) to determine if the markup needs to be processed, e.g.:
To be treated as a content file, with layout, shortcodes etc., it must have front matter. If not, it will be copied as-is.
The markup identifier is fetched from either the markup variable in front matter or from the file extension. For markup-related configuration, see Configure Markup.
Some of the formats in the table above needs external helpers installed on your PC. For example, for Asciidoc files, Hugo will try to call the asciidoctor or asciidoc command. This means that you will have to install the associated tool on your machine to be able to use these formats. (See the Asciidoctor docs for installation instructions).
Hugo passes reasonable default arguments to these external helpers by default: