We are writing a collaborative paper that will include various examples of data types. When stating the data type file extension, how should it be presented?

For example, "The platform will be able to process multiple image formats such as..."

  • jpg, tiff, gif, png (lowercase, no periods)
  • JPG, TIFF, GIF, PNG (uppercase, no periods)
  • .jpg, .tiff, .gif, .png (lowercase, periods)
  • I'd follow Wikipedia: en.wikipedia.org/wiki/JPEG ... also, they are all abbreviations, so write them as such.
    – Erk
    Mar 19, 2021 at 18:04
  • @Erk: Even if it might somewhat work in the particular cases listed in the question, note that there is a difference between "name of the file format" and "file extension". In the latter case, it is also not adviseable to deviate from whichever casing is typically found in actual usage on real files of that type. Mar 21, 2021 at 1:05

3 Answers 3


The filename is irrelevant. If I have my resume as a Word document, and I rename it to bohemian-rhapsody.jpg, then it is still my resume and not a recording of Bohemian Rhapsody, and it is still a Word document, not a "JPEG" file.

For example, "The platform will be able to process multiple image formats such as..."

If you want to talk about the image formats, then talk about the image formats, not about the filenames. But be sure to make it clear what you are talking about: the compression algorithm, the bitstream format, the container format, the file format, the metadata format?

Let's look at the kind of files that are typically named .jpg and are typically called "JPEG". "JPEG" is actually not the name of the format. It is the name of the group of people who defined the format: the Joint Photographics Expert Group.

The standard they defined is technically not called "JPEG", it is called "ISO/IEC 10918-1:1994 — Information technology — Digital compression and coding of continuous-tone still images: Requirements and guidelines".

However, this standard only defines the compression algorithm and the bitstream format. You still need to define a way how to store this bitstream into a file. The most common way of storing ISO/IEC 10918-1:1994-compressed image bitstream data as a file is the JPEG File Interchange Format (JFIF), defined in "ISO/IEC 10918-5:2013 — Information technology — Digital compression and coding of continuous-tone still images — Part 5: JPEG File Interchange Format (JFIF)".

It is important to note that JFIF is not the only file format for storing JPEG-compressed image bitstream data. There are other formats, e.g. SPIFF and JNG. So, if you only specify "JPEG", when in reality, you only handle JFIF and not SPIFF or JNG, that could lead to confusion.

Generally, you will also want to store metadata about the image itself inside of the same file. The most popular format for this is "JEITA CP-3451E – Exchangeable image file format for digital still cameras: Exif Version 2.32", commonly known as "Exif".

So, one way of phrasing it would be something like

The platform will be able to process multiple image formats such as JFIF1-encapsulated JPEG2 images with Exif3 2.32 metadata, …


  1. JPEG File Interchange Format (JFIF) as defined in ISO/IEC 10918-5:2013 [JFIF].
  2. JPEG compression algorithm and bitstream as defined in ISO/IEC 10918-1:1994 [JPEG].
  3. Exchangeable image file format for digital still cameras as defined in JEITA CP-3451E [EXIF].

Then you would properly cite those specifications in your bibliography


  • [JFIF]: ISO/IEC 10918-5:2013 — Information technology — Digital compression and coding of continuous-tone still images — Part 5: JPEG File Interchange Format (JFIF), Standard, International Organization for Standardization, Geneva, CH.

Or whatever citation style you are using.

That way, it is 100% clear what, precisely the platform is supporting, and where I can look if I want to know whether my files will be compliant with what your platform is supporting.

The same applies to PNG, which is published with identical wording both as ISO/IEC 15948:2004 — Information technology — Computer graphics and image processing — Portable Network Graphics (PNG): Functional specification as well as "W3C Technical Recommendation – Portable Network Graphics (PNG) Specification (Second Edition)".

TIFF and GIF don't have international standards. TIFF is specified by Aldus (now Adobe), and GIF by CompuServe (now long extinct, W3C maintains a copy).

  • One upvote for thoroughness. Nice job.
    – RobJarvis
    Mar 23, 2021 at 12:50
  • I was only using image extensions as example, but your use of all caps in your explanation very well illustrates the format I was looking for. Thanks.
    – terrorclaw
    Mar 23, 2021 at 16:47

According to the CMOS Online, use uppercase with no periods:

Q. How should we handle file extensions like PDF (portable document format, an Adobe Acrobat file)—lowercase, preceded by a period, or all uppercase? Other examples are GIF and JPG (or JPEG).

A. We consider these to be initialisms or acronyms, as the case may be, when they are not doing duty as file extensions appended to a file name: so PDF, JPG, GIF.


Depends on context. In most user documentation, file formats are acronyms/initialisms without special formatting. File names with extensions, or extensions standing alone are formatted the same as file names. Some publications use italics, bold, or another typographic signal to set off file names and extensions. Quotes can be used for exact names without special formatting. Here are some examples (Italics used only to set off the exmaples):

A Portable Document Format (PDF) file.

The program will export the file in PDF format with a sequential numeric name, such as "12345.pdf".

All log files will have the extension ".xml" (for Extensible Markup Format).

JPEG files can have various extensions, including ".jpg" and ".jpeg".

HTML is a markup language, and not a file format, per se. The file format is actually plain text, with the extension "htm" or "html".

Keep in mind that computer systems other than Windows can have different file naming conventions and "extensions" might not be used. Early Macintosh computers had no concept of file extensions (they had their own weirdness, "forks").

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