Excel cannot open the file 'XXXX.xlsx' because the file format or file extension is not valid.

zguzman1795 

Do you have the option of opening the file anyway? That is, is the message you're getting a warning, or is it saying you absolutely cannot open the file?

I ask because I get the following from one particular file (which I download from a reputable financial services firm), and it is just a warning; I'm still able to open it, but I'm obviously assuming responsibility for doing so.

Your message asks you to "verify that the file has not been corrupted" which seems similar, although the rest of the message is definitely more ominous.

And what did you do last July? Did you end up having to re-create the file?