reftex-file-extensions

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

(("tex" ".tex" ".ltx") ("bib" ".bib"))

Documentation:
Association list with file extensions for different file types.
This is a list of items, each item is like: (TYPE . (DEF-EXT OTHER-EXT ...))

TYPE: File type like "bib" or "tex".
DEF-EXT: The default extension for that file type, like ".tex" or ".bib".
OTHER-EXT: Any number of other valid extensions for this file type.

When a files is searched and it does not have any of the legal extensions,
we try the default extension first, and then the naked file name.

If you are using AUCTeX, you also need to add new extensions to
TeX-file-extensions.

You can customize this variable.