GridMarket Error When Uploading Header Image

Hello,

When uploading a header image of 980 x100 (and I even tried smaller images)
an error message appears that the file is too large or does not have a valid .json extension. It's strange for jpg and png files to trigger that message.

The uploaded images save and display in the admin area and on the site ok, but the error message is annoying.

Any chance of a fix for this issue?

Phil D