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diff --git a/content/22/index.md b/content/22/index.md new file mode 100644 index 0000000..c3ece23 --- /dev/null +++ b/content/22/index.md @@ -0,0 +1,52 @@ ++++ +date = 2024-09-28T18:23:12+02:00 +title = "MIME type subclassing and its consequences" + +[taxonomies] +tags = ["TIL"] + +[extra] +related = [] ++++ + +The freedesktop.org [shared MIME-info database +spec](https://specifications.freedesktop.org/shared-mime-info-spec/latest) says +the following in [section +2.11](https://specifications.freedesktop.org/shared-mime-info-spec/latest/ar01s02.html#subclassing): + +> A type is a subclass of another type if any instance of the first type is +> also an instance of the second. For example, all `image/svg+xml` files are +> also `application/xml`, `text/plain` and `application/octet-stream` files. +> Subclassing is about the format, rather than the category of the data (for +> example, there is no 'generic spreadsheet' class that all spreadsheets +> inherit from). +> +> Some subclass rules are implicit: +> - All `text/*` types are subclasses of `text/plain`. +> - All streamable types (ie, everything except the `inode/*` types) are subclasses of application/octet-stream. + +So far so good; this makes intuitive sense and seems sensible enough. There is +an interesting consequence of this rule when the MIME-info database is used by +desktop systems for file associations, however: **An application associated with +`application/octet-stream` will automatically be associated with all streamable +types as well.** + +This means that if you associate `application/octet-stream` with your text +editor, your desktop system will also suggest you open video and audio files +with that same text editor. This behaviour can be quite surprising, especially +if the association was added automatically when a file was opened through the +"Open with..." dialog. + +What is even more confusing if you don't happen to know the subclassing rule is +the fact that `~/.config/mimeapps.list` and applications interfacing with this +file will not even list the editor as associated with any audio or video files. +You might just skip over the entry it has for `application/octet-stream`, not +realizing its significance. Perhaps you even assume (understandably) that +`application/octet-stream` only specifies any file of "unknown" type. +User-facing documentation on desktop systems (if it even exists) does not +discuss this behaviour. + +Whilst looking into this I found an older KDE bug report with some [interesting +thoughts](https://bugs.kde.org/show_bug.cgi?id=425154#c2) on how to explain this +behaviour to the end user, but sadly as far as I have seen none of these have +made it into the system setting's file association dialog. |