A new PNG spec

> there is no way for a user to tell from the look of the port or the file extension what the capabilities are

But that's typical for file extensions. Consider EXE – it is probably an executable, but an executable for what? Most commonly Windows – but which Windows version will this EXE run on? Maybe this EXE only works on Windows 11, and you are still running Windows 10. Or maybe you are running x86-64 Windows, but this EXE is actually for ARM or MIPS or Alpha. Or maybe it is for some other platform which uses that extension for executable files – such as DOS, OS/2, 16-bit Windows, Windows CE, OpenVMS, TOPS-10, TOPS-20, RSX-11...

.html, .js, .css – suggest to use a web browser, but don't tell you whether they'll work with any particular one. Maybe they use the latest features but you use an old web browser which doesn't support them. Maybe they require deprecated proprietary extensions and so only work on some really old browser. Maybe this HTML page only works on Internet Explorer. Maybe instead of UTF-8 it is in some obscure legacy character set which your browser doesn't support.

.zip – supports extensible compression and encryption methods, your unzip utility might not support the methods used to compress/encrypt this particular zip file. This is actually normal for very old ZIP files (from the 1980s) – early versions of PKZIP used various deprecated compression mechanisms, which few contemporary unzip utilities support. The format was extended to 64-bit without changing the extension, there's still a lot of 32-bit only implementations out there. ZIP also supports platform-specific file attributes–e.g. PKZIP for z/OS creates ZIP files which contain metadata about mainframe data storage formats, unzip on another platform is going to have no idea what it means, but the metadata is actually essential to interpreting the data correctly (e.g. if RECFM=V you need to parse the RDWs, if RECFM=F there won't be any)

.xml - okay, it is XML – but that tells you nothing about the actual schema. Maybe you were expecting this xml file to contain historical stock prices, but instead it is DocBook XML containing product documentation, and your market data viewer app chokes on it. Or maybe it really is historical stock prices, but you are using an old version of the app which doesn't support the new schema, so you can't view it. Or maybe someone generated it on a mainframe, but due to a misconfiguration the file came out in EBCDIC instead of ASCII, and your app doesn't know how to read EBCDIC, yet the mainframe version of the same app reads it fine...

.doc - people assume it is legacy (pre-XML) Microsoft Word: every version of which changed the file format, old versions can't read files created with newer versions correctly or at all, conversely recent versions have dropped support for files created in older versions, e.g. current Office versions can't read DOC files created with Word for DOS any more... but back in the 1980s a lot of people used that extension for plain text files which contained documentation. And it was also used by incompatible proprietary word processors (e.g. IBM DisplayWrite) and also desktop publishing packages (e.g. FrameMaker, Interleaf)

.xmi – I've seen this extension used for both XML Model Interchange (XML-based standard for exchanging UML diagrams) and XMIT (IBM mainframe file archive format). Because extensions aren't guaranteed to be unique, many incompatible file formats share the same extension

.com - is it an MS-DOS program, or is it DCL (Digital Command Language)?

.pic - probably some obscure image format, but there are dozens of possibilities

.img – could be either a disk image or a visual image, either way dozens of incompatible formats which use that extension

.db – nowadays most likely SQLite, but a number of completely incompatible database engines have also used this extension. And even if it is SQLite, maybe your version of SQLite is too old to read this file because it uses some features only found in newer versions. And even if SQLite can read it, maybe it has the wrong schema for your app, or maybe a newer version of the same schema which your old version that app doesn't support, or an old version of the schema which the current version of the app has dropped support for...

Just last week I had again some PDFs Okular could not open because of some more uncommon form features.

> Consider EXE – it is probably an executable, but an executable for what? Most commonly Windows

Has anyone ever used .exe for anything other than Windows?

Way back when, my prof was using his Linux machine to demonstrate how to use GCC. He called the end result .exe but that might have been for the benefit of the Windows users in the room. (Though Linux users being considerate to Windows users, or vice versa, is admittedly a rarity)

Prior to Windows 95, the vast majority of PC games were MS-DOS exe files – so anyone who played any of those games (whether back in their heyday, or more recently through DOSBox) has run an MS-DOS exe. Most people who ever used Lotus 1-2-3 or WordPerfect were running an MS-DOS exe. Both products were eventually ported to Windows, but were far less popular under Windows than under DOS.

Under Windows 95/98/Me, most command line tools were MS-DOS executables. Their support for 32-bit Windows console apps was very poor, to the extent that the input and output of such apps was proxied through a 16-bit MS-DOS executable, conagent.exe

First time in my life I ever used GNU Emacs, it was an OS/2 exe. That's also true for bash, ls, cat, gcc, man, less, etc... EMX was my gateway drug to Slackware

> Has anyone ever used .exe for anything other than Windows?

Did you know that Microsoft Windows originally ran on top of the much older MS-DOS, which used EXE files as one of its two executable formats? Most Windows users had lots and lots of EXE files which were not Windows executables, but instead DOS executables. And then came Windows 95, which introduced 32-bit Windows executables, but kept the same file extension as 16-bit Windows executables and 16-bit DOS executables.