ASM- Null in multi-part parameter value- Clarification

It's pretty clear that the filename is being considered as a parameter name, instead of 'name' and 'filename', which are the actual parameter names.

I wonder if ASM is misidentifying the parameters and values because the parameters aren't separated by an ampersand '&' character (the presence of name-value pairs separated by an ampersand is used to automatically identify form data by ASM)?

You could try adding 'name' and 'filename' as parameters in your policy, then allow the null (or space) character as an acceptable metacharacter in the parameter value.