Get file content

Honors the Accept header. If the value of the header is text/plain and there is a document in the index that has the genre detected as plain text, the contents of the document will be returned. If the document is not found, HTTP error 404 will be returned. If the genre of the document is not plain text, HTTP error 406 will be returned. Alternatively, use application/octet-stream to bypass the document and genre check. The Content-Type header of the reply will be set accordingly.