@param buffer The kstring to be used to store the modified filename
@param filename The filename to be (copied and) adjusted
@param replace If non-zero, one extension (if any) is removed first
@param extension The extension to be added (e.g. ".csi")
@return The modified filename (i.e., buffer->s), or NULL on error.
@since 1.10
If _filename_ is an URL, alters extensions at the end of the hier-part,
leaving any trailing ?query or #fragment unchanged.
@param buffer The kstring to be used to store the modified filename @param filename The filename to be (copied and) adjusted @param replace If non-zero, one extension (if any) is removed first @param extension The extension to be added (e.g. ".csi") @return The modified filename (i.e., buffer->s), or NULL on error. @since 1.10
If _filename_ is an URL, alters extensions at the end of the hier-part, leaving any trailing ?query or #fragment unchanged.