Download a sandbox file

Download file contents from a sandbox filesystem path. Supports HTTP range requests: send a Range header (for example bytes=0-1023) to receive a 206 with only that byte range. Every response carries an ETag; send it back as If-Range to resume safely (a changed file returns 200 with the whole file instead of mismatched bytes) or as If-None-Match to get a 304 when the file is unchanged. HEAD returns the same headers, including the file’s size in Content-Length, without the body.