There is only one file type that uses the .TMX extension:
Translation Memory Exchange File
File created in the Translation Memory Exchange (TMX) format, an open XML standard used for exchanging translation memory (TM) data created by Computer Aided Translation (CAT) and localization applications; may save words or phrases that have been translated from one language to another; used for transferring the translation memory between different tools and vendors.
NOTE: The TMX format was developed by the Localization Industry Standards Association (LISA), who then released the standard under the Creative Commons license.
Updated: June 11, 2020