Hi @yanokwa and @Grzesiek2010
Thanks for your replies - glad it wasn't just me being unable to read the manual!
I'm sure that this is a daft suggestion, but this almost works already with 'choose image' in the camera widget (except of course it fails as an unsupported file type), so perhaps there is code almost ready to go.
If you are considering this as a new feature it would be great to be able to specify the supported file type(s), which will help the collector - this could be an attribute or constraint? If you could filter out files based on type that would be a great safety net
I can't really see why you'd need to limit file size (I know some people would be worried about bandwidth and storage in aggregate), but could that be another constraint in the settings. There would be nothing more frustrating in the field than having a file very slightly too big to include (and not have the tools available to edit it!). You could put a message on collector if the file is over a given size asking for confirmation?
Appreciate your positive approach to problems! They become opportunities 
Thanks.