Marcel is dead on with this, but I'll add a bit more from a best practice perspective.
When I hit the Get Files From Folder, I'll do a couple of things:
1) change the file extension type to all lower case
2) filter the file type to ONLY the file type I'm expecting to work with
3) filter out any subdirectories I do not want to combine
4) filter out any files that start with ~$ (these are temp files)
5) add any other required filters to get down to just the files I need
I ALWAYS do the first two, just as a matter of good practice (in case some joker throws a Word doc in the folder), but the others can be applied as needed in order to protect against the issue you're experiencing.
I then name that query "FilesList", create a new query by referencing my FilesList query, and combine from there. This practice will set you up best as you roll into the new
Combine Binaries experience that may/may not have hit your version of Excel just yet.