I like the iA Writer syntax. In the link @james posted, there’s some further examples for transcluding text, tables, and images:
/Section.txt "Section"
/Balance Sheet.csv 'Finances'
/images/Structure.jpg (Data Flow)
The use of a forward slash for all transcluded file types is easy to remember. I think the iA Writer syntax is even more intuitive for causal writers than Markdown’s regular image syntax.
In some ways this syntax is reinventing the wheel. In Markdown we already have a syntax for including images, which could be extended for other embedded files. For the iA Writer examples, we could use this syntax:
However, the text in brackets or quotes in the iA Writer syntax is displayed as a caption - whereas in Markdown, alternative text is not displayed as a caption without some non-standard post-processing. So there might be a case for using the iA Writer syntax for embedded content with optional captions, rather than hijacking the Markdown image syntax reserved for alt or title to display figures.
There’s also the question of how this information should be transcluded. For example, iA Writer presents the CSV file as a table. So there would need to be file type specific rules regarding transclusion. This follows on from the embedded audio/video discussion.
As CommonMark extensions, would there be harm in supporting both? Markdown has always followed TMTOWTDI.