wpostx files are rare however they are sometimes found as blog attachments, they are apparently related with Windows Live Writer.
If you would like to open a wpostx file you can try to rename it to wpost and try to open it using Windows Live Writer, however we cannot assure that will show the file properly.
If you have more details about wpostx files please let us know by using the Contact link on bottom of this site.
How to open .wpostx files?
Software to open .wpostx files
What are .wpostx files