Look at the "General" and see what "Kind:" says about the file. Also look at "Name & Extension:".
If the file extension ends with .jpg.mix then all you need is a tool to "rename" (because doing them all by hand is a chore).
You want to remove the .mix and leave the .jpg extension.
If the file name ends with .mix follow these steps:
Switch to the Finder and open a folder that has these files. Rename the .mix to .jpg and a dialog window will open asking to confirm.
If you have hundreds of files a batch rename is in order. Terminal can do it and other software is available to batch rename.