[lmm]

The user presumably wants to know what kind of content they're going to receive from a given endpoint - foo.html, foo.pdf, foo.jpg, foo.mp3 and foo.avi suggest quite different experiences, and it's nice to include that hint in the URL (where it's visible on mouseover) rather than the user having to go in blind. I also like being able to reassure the user that they're receiving the same piece of content however they access a given resource, rather than the possibility of invisible content negotiation changing the site's behaviour.

[neilalexander]

Such hints can be unreliable at best and misleading at worst. There is nothing to guarantee that the file extension and the Content-Type header will agree, nor is there anything to guarantee that the file name in the URL will match the download file name in the Content-Disposition header.

[lmm]

Well, sure, but you can just... not do that? The <title> tag can be misleading because there's no guarantee that the title matches the content of the page - but the answer to that is to use good titles for your pages, not to avoid using the <title> tag.

[neilalexander]

My point is that it isn’t really reassuring for the user because of invisible negotiations. If anything, I would lean more towards guessing that users are, more often than not, either ignorant or untrusting of URL contents, either because the URLs so frequently look like nonsense (arbitrary content IDs instead of meaningful names) or because they have already proven to be unreliable elsewhere on the web (deep links not as deep as expected when copied or shared).

[lmm]

> If anything, I would lean more towards guessing that users are, more often than not, either ignorant or untrusting of URL contents, either because the URLs so frequently look like nonsense (arbitrary content IDs instead of meaningful names) or because they have already proven to be unreliable elsewhere on the web (deep links not as deep as expected when copied or shared).

I think users are (rightfully) distrustful of URLs in the general case, but a URL having a file extension is actually a pretty good indication that it's a simple "does what it says on the tin". (Imgur changing their .jpg URLs to have more complex behaviour caused a pretty big backlash, for example)