We can do this now but not with the specified content-type, as we are returning 415 but not with a specific rejected content type.

So for now I would return something like "Unable to cite unsupported media types such as pdfs, image and movie files."

Proposal:
We would have the following message with the first sentence in bold red-error color token with the appropriate error icon, followed by the description in base-black color and a link to documentation on Citoid's supported formats.

We couldn’t create a citation for you.
Learn more about the formats we support
or create one manually below.

Mockup:

I find this message kind of confusing because the statements aren't explicitly connected.

"We couldn't make a citation for you because the resource is in a format we don't support" sounds clearer to me.

In T369547#10063836, @Mvolz wrote:

I find this message kind of confusing because the statements aren't explicitly connected.

"We couldn't make a citation for you because the resource is in a format we don't support" sounds clearer to me.

Great spot, @Mvolz . What do you think about the proposed version below?

Separately: what page do you think should be linked from the format we don't support portion of the message?

We couldn't create a citation for you.<br>
The resource is in a [[TBD | format we don't support]]. Create a citation manually below.

In T369547#10079818, @ppelberg wrote:

In T369547#10063836, @Mvolz wrote:

I find this message kind of confusing because the statements aren't explicitly connected.

"We couldn't make a citation for you because the resource is in a format we don't support" sounds clearer to me.

Great spot, @Mvolz . What do you think about the proposed version below?

Separately: what page do you think should be linked from the format we don't support portion of the message?

We couldn't create a citation for you.<br>
The resource is in a [[TBD | format we don't support]]. Create a citation manually below.

Fine but somewhere along the way the examples of what we mean by "format" have been lost. We need to give an example like "pdf" or else I don't think people will understand.

https://gerrit.wikimedia.org/r/c/mediawiki/extensions/Citoid/+/1084841

⚠️ "Create" button does not have a visible focus state
⚠️ "Create" button is not disabled when there is an error, per expected behaviour from mockup. As a result, users are able to continue retrying

Thanks @EAkinloose, that's really helpful. It looks like the Citoid service might be returning 404 on beta cluster. I'm seeing a 404 error from the REST API:

https://en.wikipedia.beta.wmflabs.org/api/rest_v1/data/citation/mediawiki/https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F9%2F98%2FColoring_page_for_Wikipedia_Day_2019_in_NYC.pdf?action=query&format=json

That means we never see a 415 error, and therefore never see the more specific error text. @Mvolz is this an expected issue on beta cluster?

In T369547#10323793, @dchan wrote:

Thanks @EAkinloose, that's really helpful. It looks like the Citoid service might be returning 404 on beta cluster. I'm seeing a 404 error from the REST API:

https://en.wikipedia.beta.wmflabs.org/api/rest_v1/data/citation/mediawiki/https%3A%2F%2Fupload.wikimedia.org%2Fwikipedia%2Fcommons%2F9%2F98%2FColoring_page_for_Wikipedia_Day_2019_in_NYC.pdf?action=query&format=json

That means we never see a 415 error, and therefore never see the more specific error text. @Mvolz is this an expected issue on beta cluster?

I don't know how citoid is updated on beta. I know it was never set up with Zotero running behind citoid, so it's not identical to production. It's only recently been adopted by a team (https://phabricator.wikimedia.org/T215217) after being basically stale for years.... I assume that it's just running an old version of citoid from before we implemented 415 (which was months ago). The citoid in prod is updated manually by me. I did find this ticket. T346624

But a youtube video would work anyway, because a youtube link is html, NOT a video link... maybe we need to clarify language more as it's too technical? Instead of "movies", maybe .pdf, .mov, .mp3?