Prevent smarty from caching the Dynform rendering result.

I was pretty sure I'd confined the bits that turn on smarty caching to the frontend code. (Because this is the only place that needs it.) So this fix would not be necessary. Yet here I see a recent system building a smarty instance with caching already turned on. Has that commit fallen under the table in the last year or what? To be investigated.
Fix size-modifier to use FILEBASEDIR instead of working from the current dir (which could be anywhere).

Also write 'kB' to use the proper SI-prefix and have 'MB' in decimal (10^6) instead of 1000*1024. Srsly wtf, we were using the floppy bastard Megabyte where 1.44MB meant 1440*1024 Byte. So wrong.