Read media information (image / song / video) in SendFileBox in the
same way it is read in FileLoadTask::process() and reuse it in
FileLoadTask if it was loaded already for the box.
Now base::lambda can be copied and can wrap any immutable lambda.
For mutable lambdas there is base::lambda_once (which you're
supposed to call only once to pass data through lambda captures,
for example by using std::unique_ptr). Generally base::lambda is
passed by value and base::lambda_once is passed by rvalue reference.
John Preston