Ah, I think I sorta misunderstood the issue. I apologize for reading too quickly. Anyways, mailcap is “mail capability”, it provides the MIME type lookup table that the Go standard lib uses. At one point, it was missing from the docker image so some users were running into issues with content types.
Your question is different in that it relates more specifically to the charset. As an aside, I’ve always understood that as being optional. Where is that causing issues for you exactly, out of curiosity?
One fix would be to override the header if the request is for a .json file:
@isJson path *.json
header @isJson Content-Type "content-type: application/json; charset=UTF-8"