File uploader tells "you can only updload the following extensions" even if the extension is correct

I had two cases today of people telling me they couldn't upload an image using a file upload widget even if the extension and size of the file was correct (The message is the one shown in the image).

To correct this issue (the first time) I edited the file extensions allowed by the file uploader to only "jpg, jpeg, png, gif" and saved the form. It was solved, but it appeared again. I edited the form with the intention of just saving it (i.e.: edited some text in an unrelated field) and the problem solved.

Some extra info: I cloned this form from an existing one, it has been working well until today.

Hello, this is in reference to the other thread where you replied to. I have now cleared your Form Cache so please give it a spin and let us know how it goes.

BTW, you may also clear your Form Cache from your end, just refer to this short guide: How-to-Clear-Your-Form-Cache