I would say it's text when interpreted as text/plain it's human readable. Otherwise it's binary. That is, binary = for machines only.

the subversion FAQ http://subversion.tigris.org/faq.html#binary-files has = " ... if any of the bytes are zero, or if more than 15% are not ASCII printing characters, then Subversion calls the file binary. This heuristic might be improved in the future, however."

I couldn't find a definition of text except that text means absence of binary data. This is weak - so I would follow your definition - A text file is a file which can be read by a human.

The distinction doesn't refer to the files _contents_ but how to the file is _treated_ when it is being read or written. In "rb"/"wb" modes files are left how they are, in "r"/"w" modes Windows programmers get line ends "\r\n" translated into "\n" what disturbs file positions and string lengths.

Dividing files into "text" and "binary" is the archetype misdesign in the operating system you use