[LyX/master] Explain why the binary detection works

[Georg Baum]

commit 4bea5b9d18af249661e23c462fea3f59042bac03
Author: Georg Baum <b...@lyx.org>
Date:   Sat Jun 7 17:48:23 2014 +0200

    Explain why the binary detection works
    
    Extracted from Enricos comment at bug 9146.

diff --git a/src/Format.cpp b/src/Format.cpp
index ff7fb34..44e3f63 100644
--- a/src/Format.cpp
+++ b/src/Format.cpp
@@ -379,6 +379,12 @@ string guessFormatFromContents(FileName const & fn)
                                maybelatex = true;
                }
 
+               // Note that this is formally not correct, since count_bin_chars
+               // expects utf8, and str can be anything: plain text in any
+               // encoding, or really binary data. In practice it works, since
+               // QString::fromUtf8() drops invalid utf8 sequences, and while
+               // the exact number may not be correct, we still get a high
+               // number for truly binary files.
                binchars += count_bin_chars(str);
        }