Is it correct to use the contraction in Your file'll be downloaded...? I don't know. It just doesn't sound right to me. English is not my first language, so maybe someone here could shed some light on this issue.
The construct is an awkward one, due to the absent (contracted out) vowel, but entirely legal.
Native English speakers know to insert a schwa in place of the absent vowel when speaking.
As noted elsewhere, though grammatical this particular contraction should perhaps be avoided.
It's an acceptable contraction as it makes grammatical sense.
However, given the assumed context (a message on a website?), I would personally avoid it and go with "Your file will...".