I need help. I have an issue with the not correct value of thumbnail_url in response for /Rest/Media/1/Files. For exmaple: /Rest/Media/1/Files/?ids=47166705&result_columns=id&result_columns=thumbnail_url
Thank you for pointing this out. This appears to be a bug. The value of thumbnail_url is dynamic and should reflect the default size of 500. The value of 400 is a legacy value from Fotolia that is no longer valid.
There is an easy workaround while you are waiting for this to be patched. Instead of requesting a generic thumbnail size, request a specific size such as thumbnail_500_url. These values are documented in the Search API here: