Files can be referred to in several ways:
- by description (eg Microsoft Word document)
- by accepted abbreviation (eg PDF)
- by file extension (eg .docx in document.docx).
Which is best depends on what is being communicated. A generic term (eg bitmap image) may not be specific enough; many bitmap image file formats exist. Giving the acronym (eg BMP, PNG) is more precise.
In general discussions, it may be best to use the description followed by the acronym on first use, unless the format is familiar to the audience:
Copy the DOCX files to the backup folder.
All the crystallographic information files (CIFs) should be checked using EnCIFer.
Treat the abbreviation in this way whether the extension is a genuine acronym or initialism (eg PDF – portable document format) or not (eg XLSX – Excel spreadsheet).
In some contexts – for example, when giving detailed instructions about commands to be typed – the file extension is a useful way to identify file types. This is because it is what would be typed in an actual command. When referring to the file extension, use lower case and the full stop. If the discussion includes code listings or examples of program output (see Code and Typing commands), consider using the same monospaced font for all file names, parts of names, output and code:
Copy the .docx files to the backup folder. Use the following command:
copy *.docx c:\backups
Copy the .bmp files to the backup folder (c:\backups). Be sure to include icons.bmp.
Did you know? Common monospaced fonts include Courier New, Consolas, Inconsolata, Source Code Pro and Lucida Console, but there are many. Find one that visually matches the main body font. This may mean using a font family that includes proportional and monospaced fonts, or experimenting with font choice and size to get a good match. Monospaced fonts often look too large when used at the same point size as the body font.
Be aware that the same file type can take different extensions (eg .jpg versus .jpeg). Make sure the reader knows precisely which files you are talking about.
