looks up file size in the header as well as last modified

[jenningsanderson]

Building on the previous update to lookup the "Last Modified" date, this also looks up the file size so that it doesn't get out of date.

@tyrasd - do you see any problems with this? It's similar to what you proposed before... is there any reason this might stop working?

[tyrasd]

AFAICT, not all browsers consider the Content-Length HTTP header as CORS safe yet (Firefox version 78 for example: https://developer.mozilla.org/en-US/docs/Glossary/CORS-safelisted_response_header, see screenshot below). The header is however already in the fetch specs as a safelisted header: https://fetch.spec.whatwg.org/#cors-safelisted-response-header-name, so it will probably just take some time until it gets included in Firefox as well (I guess).

To make this work currently in all browsers, the S3 bucket would need to return the CORS expose-headers header in the result (i.e. Access-Control-Expose-Headers = Content-Length), as mentioned already in #26 (comment). Alternatively, one could just ignore null values in these cases and keep the default value (e.g. "40GB").

[tyrasd]

@dakotabenjamin can you set up the S3 bucket's ´Access-Control-Expose-Headers` CORS header?