How do you set the default app to open a Text/Plain attachment?

Good idea, and it works as a specific solution for the suffix. Attachments with the same suffix will open on double click with TextEdit but text/plain attachments with a different suffix still require my selecting TextEdit to open the attachment.

I believe the association has to be defined in Mail for mediatype, so that all files of the text/plain mediatype will be opened with TextEdit (see https://developer.mozilla.org/en-US/docs/Web/HTTP/Basics_of_HTTP/Data_URls for MIME mediatypes)

Mail handles text/html, base64, image/ etc fine but for some reason or other it croaks on text/plain which is the default MIME mediatype.

That Mail does not process text/plain correctly seems more like a deliberate decision rather than an oversight, so am hoping someone know of a setting somewhere which can be changed/modified.

"The operating system decides what file extensions are openable by TextEdit...."

My question has nothing to do with the operating system or file extensions, it is email.

The email application decides what to do with an attachment according to the MIME mediatypes defined by an Internet Assigned Numbers Authority (IANA) standard. See https://en.wikipedia.org/wiki/Media_type

The Mozilla page was referenced because it has a succinct explanation of what Text/Plain is.

Mail is the only email application I use which does not open Text/Plain emails with a text editor on double click.

Some applications don't even require you to double click the attachment. Thunderbird will open Text/Plain attachments with a text editor if you want to double click but you don't have to, the application presents the text below the attachment's name.

Mail handles other Mime mediatypes correctly, but it does not open Text/Plain on double click on my machine under El Capitan.