Allow str in form files.

[waterfountain1996]

trafficstars

Closes: #2069

Changed FileField to allow str content, but raise exception on text buffers.

[lovelydinosaur]

Great - thanks!

I've tweaked the description to "Closes: #2069", so that the issue gets a backlink to here, and so that merging this would auto-close the issue. Also retitled the PR, dropping the issue number.

I'm... not sure which of the two possible resolutions suggested in #2069 we should go for.

Possibly(?) in favour of making the docs change, rather than making the behaviour change.

That would still give us the option of making the behaviour change at a later date if we think that's a good thing to do, because adding str support there isn't be a breaking API change. (But removing it is.)

Not sure.