“Not a valid image” when trying to upload images in Joomla! 3.7

Error message “Not a valid image” displayed when trying to upload an image in Joomla! 3.7. The error message is displayed even though the file type being uploded (jpg or png for example) is existing as valid type in both valid file types and mime types fields.

Solution: Go to System -> Global configuration -> Media and select Check MIME types = No