Say I have a email attachment in Thunderbird (Mac) with the extension .log. If I right-click that and select Open, a window pops up asking what Thunderbird should do with the file, and under Open, the only options are "TextEdit (default)" or "Other...". Where does this TextEdit default come from? In the Thunderbird settings, General panel, under Files & Attachments, I've specified that text files should be opened with BBEdit.
Not an apple user here, but Google tells me TextEdit is a built in component of the apple operating system and is the default text handler for that operating system.
You specified plain Text files, which are identified in the email using their mimetype. I don't doubt that a log file will be encoded as text, even if that would be more correct. I suggest your check in the message source what mime encoding is specified. Here is a list of registered text attachment types. https://www.iana.org/assignments/media-types/media-types.xhtml#text