add button to calculate checksums if file > 250MB

[ericpyle]

No description provided.

[mvahowe]

When we get to this, I think I'd go for a cut-off of 50Mb. That should mean that pretty much any audio gets checksummed and, also, it should happen fast enough to not create the "why has everything stopped and why is my laptop catching fire?" sensation I experienced the first time I added video resources.