+ (**
+ @param fname name of the file to be sent
+ @param contype Content-Type header value
+ @param contenc Content-Enconding header value
+ @param patch_fun function used to patch file contents
+ @param gunzip is meaningful only if a patch function is provided. If gunzip
+ is true patch_fun is applied to the uncompressed version of the file. The file
+ is then compressed again and send to client
+ @param outchan output channel over which sent file fname *)