Case made well by @erasche on #1746, fixes#1746.
To resolve this more correctly (and many other potential type problems) - the workflow editor should be significantly reworked to track the datatype (or a range of datatypes) on each terminal (the way it does for collection mapping information). The change would be a weeks worth of development effort and inappropriate to apply to a release branch.
Long term this could allow Galaxy to support - multiple tooling formats (Galaxy-like YAML, CWL http://bit.ly/cwltooldesc, etc...). But I think it is also important from a purely design perspective - this is a core logic class integrating different components - they should not also be doing XML parsing.
To verify the interface for parsing tools is expressive enough to allow multiple useful implementations, I built a test YAML tool description that implements many of the same features as Galaxy but smooths out rough edges (uses exit codes for job failure by default for instance). Loading these tools is disabled by default and it is not documented how to enable them because they are not intended to be part of Galaxy's public API.
Directly setting format attribute, setting to format to 'input' (ambigious, non-deterministic and should be deprecated IMO), using format_source, and using change_format actions.
Nicola Soranzo