If Vim doesn’t recognize the filetype of the currently edited file, I can tell Vim what filetype to use. Consider, for instance, that I have a draft of a markdown file with the name, documentation.md.draft. Vim will not recognize this as a markdown file and will, thus, not apply markdown syntax highlighting to that file. I can easily tell Vim to treat this as a markdown file by setting its filetype:

:set filetype=markdown

Markdown syntax highlighting and other relevant options will now be applied to the current buffer.

See :h filetype for more details.