[ WIP/ Needs discussion ] handle all binaries with a big constant

[KeynesYouDigIt]

Closes #405

Why is this the best possible solution? Were any other approaches considered?

This solution centralizes the handling of Sandbox files in a form while adding support for multiple binary default types.

What are the regression risks?

Users attempting to add sandbox files that don't conform to expectations might see unexpected behavior - I have tried to avoid this but its worth discussing how common it is.

In testing, I found a few CSV and XML handling bugs, but those were cleared up. Still, I dont know if tests are missing for these and other file types being used in an unexpected way.

Overall, my question is: do we have validation on paths to binary files? if not, is now the time to add it? or could we choke out some much needed flexibility?

Does this change require updates to documentation? If so, please file an issue here and include the link below.

Only if we add stricter validation of file types, and even then only maybe.

Before submitting this PR, please make sure you have:

• included test cases for core behavior and edge cases in tests_v1
• run nosetests and verified all tests pass
• run black pyxform to format code
• verified that any code or assets from external sources are properly credited in comments