[TYPO3-mvc] Confusion with filesize() path

[g4-lisz at tonarchiv.ch]

Hi there,

i'm using the PHP filesize function in a few places of my extension.

For the views i wrote a view helper which works like expected i.e. 
filesize() can handle the reltive file path:

     filesize('uploads/tx_myext/pdf/XXX.pdf')

If i'm using the same call directly from an action controller in the 
backend, i always get the size=0.

Why does the reltive path work in a view helper, but not in a action 
controller? Is this because it's a controller for a backend template?

Cheers,
Till