file's contents.
The contents are placed in contents, and length is set to the
size of the contents string. If etag.out is present, it will be
set to the new entity tag for the file.gFileLoadContentsFinish(object, res, .errwarn = TRUE)objectGFile.resGAsyncResult.TRUE if the load was successful. If FALSE and error is
present, it will be set appropriately.contentslengthNULL if the length is not neededetag.outNULL if the entity tag is not needederrorGError, or NULL