I think the short answer to your question is that by design, Tails itself should not attach its own metadata to files [...]

This is incorrect. While we may have disabled writing such metadata for some of the applications we ship in Tails, I'm sure there are applications that don't have such an option (so custom patches would be necessary, which is a big pain and, given our current resources and priorities, is not worth it), and for some we may have missed it. Do not assume that documents/media created using the tools bundled in Tails are free from metadata. On the bright side, since Tails has a single profile for all Tails users, that at least limits the extent of such leakage. But still, stuff like the creation time of the document, and the fact that it was created using Tails, can still leak.

[...] if you post images or photos you should be very careful to first search for and remove any metadata they may have acquired before you even transferred them to a computer running Tails.

For all document/media formats that supports it, we recommend stripping the metadata using MAT, which is included in Tails.