Why do some applications have files with no extension?

90% of the filenames circled are all uppercase letters. This is addressed in our sister-site Software Engineering:

• Readme.txt vs. README.txt

All-uppercase letters stand out and make the file easily visible which makes sense because it is probably the first thing a new user would want to look at. (Or, at least, should have looked at…) As others have already said, file names starting with a capital letter will be listed before lower-case names in ASCIIbetical sorting (LC_COLLATE=C) which helps make the file visible at a first glance.

The README file is part of a bunch of files a user of a free software package would normally expect to find. Others are INSTALL (instructions for building and installing the software), AUTHORS (list of contributors), COPYING (license text), HACKING (how to get started for contributing, maybe including a TODO list of starting points), NEWS (recent changes) or ChangeLog (mostly redundant with version control systems).

This is what the GNU Coding Standards have to say about the README file.

The distribution should contain a file named README with a general overview of the package:

• the name of the package;
• the version number of the package, or refer to where in the package the version can be found;
• a general description of what the package does;
• a reference to the file INSTALL, which should in turn contain an explanation of the installation procedure;
• a brief explanation of any unusual top-level directories or files, or other hints for readers to find their way around the source;
• a reference to the file which contains the copying conditions. The GNU GPL, if used, should be in a file called COPYING.
  If the GNU LGPL is used, it should be in a file called
  COPYING.LESSER.

Developers interested in file naming conventions should visit Software Engineering site.