Permissions in wp_content/uploads/YYYY/MM

When I upload an image to the media libray, a number of files are created in folder wp_content/uploads/yyyy/mm where yyyy = current year and mm = current month number. One has the name of the uploaded file made into a URL, the others have the file name with dimensions appended eg file_150x300.png. I have had a problem where an image in an image block refers to the file without any dimensions appended and an error 500 occurs when trying to access it from a browser. In WordPress the image is not loaded into an image block. The files with the dimensions appended do not have the same permissions as the file which does not. What permissions should be applied to all files to make them accessible to WordPress?