The general convention is <output file name> + ".in".

avformat_find_stream_info was introduced in Libav avformat 53.3,
but it was only merged to FFmpeg in avformat 53.6.

In Libav avformat 53.3-53.5 av_find_stream_info is not removed
(only deprecated), so this shouldn't break building with that.

Searching in directory names can yield confusing results; e.g. if
the input is "jpeg2000/image1.jp2", it will infer the pattern
"jpeg%04d/image1.jp2", which is likely not what the user intended.

If the user really desires for the variable part to be in the
directory name, it can always use an explicit pattern.