Transloaders were a scheme invented by Computer Concepts to allow files to be transparently translated from one format to another when loaded into software like Impression.
Support for the system was added to Ovation Pro.
The Transloader system depends on wimp messages, but there is lots of file conversion software which consists of command line tools that take an input file and produce an output file. For example ChangeFSI which converts many bitmap image formats. The TransShell program allows a command line file converter to be inserted in a shell which will let it work as a transloader.
Download RISC OS source code