reftex-try-all-extensions

reftex-try-all-extensions is a variable defined in `reftex-vars.el'.
Its value is

nil

Documentation:
Non-nil means, try all extensions listed in `reftex-file-extensions'.
When searching for a file, LaTeX uses only the default extension. However,
if you are working with a noweb system that produces the .tex files from
some other file, and you want RefTeX to scan the web file instead of the
tex file, you need to set this option. You also need to make the noweb
extension the default extension, i.e. the first in the list in
`reftex-file-extensions'.
Note that if you are using external file finders, this option has no effect.

You can customize this variable.