tex-default-mode

tex-default-mode is a variable defined in `tex-mode.el'.
Its value is

latex-mode

Documentation:
Mode to enter for a new file that might be either TeX or LaTeX.
This variable is used when it can't be determined whether the file
is plain TeX or LaTeX or what because the file contains no commands.
Normally set to either `plain-tex-mode' or `latex-mode'.

You can customize this variable.