add file prefix support, fix multiple parameter...

Hi,

I am new on github... but as I programmed some stuff I tought asking if maybe you wan't to use my changes...

1. Added file prefix support with a backend field in which one can also use the ##field_name## to get field-dependent prefixes.

2. Fixed the multiple parameter so that if initalised with "false" in the frontend-template it will be passed on, so that one has the possibility to limit uploads.

3. Do not know but when a file is chunked ... all the names of the chunk files where the same ... I changed it and appended part nurmbers.

sorry for the formatation changes ... as I said.. a bit new here :)

greetings,
Gerald