Error performing mail merge "Incompatible file type and file extension"


Following an upgrade to MS Word you may see an error relating to file type and extension:

Subsequent attempts to create attachments based on templates will fail with the message that the DTS Source files are locked.

The initial error in the screenshot above may be caused by the default file format that MS Word attempts to use for saving new files.  If this is set to OpenDocument Text *.ODT it should be changed back to Word document *.DOCX, as described in this Microsoft article: