Language issue: The name and description for the attachment maximum width should be written as "maximum width / height" instead of only width; because in the coding it is used as a maximum value for both:
Current:
"Maximum thumbnail width in pixel:" and "A generated thumbnail will not exceed the width set here."

Should be
"Maximum thumbnail longest edge in pixel:" and "A generated thumbnail will not exceed the width and height set here. Also, a thumbnail will only be generated, if the original image dimensions are bigger than the values specified here".