A _DOCX file is a Microsoft Word Open XML document with the .docx extension changed to ._docx. It is a .DOCX file, which includes formatted text, images, graphs, tables, and charts. _DOCX files may be renamed to "._docx" files by an email program to prevent the file from being blocked by an email service.
Another reason why a _DOCX file may be created by an email program, such as Windows Live Mail, is if the filename ends with a "." before the "." in the extension. For example, the email program changes sample..docx to sample._docx. This also commonly occurs with .DOC, .XLS, and .XLSX files.
You can rename _DOCX files by changing the "._docx" extension to ".docx". After renaming the file, you can open it as a standard DOCX file. Some programs that can open DOCX files are Microsoft Word, Corel WordPerfect, LibreOffice Writer, and Apple Pages.