Enable FileRef to detect file types by the actual content of a stream.

FileRef doesn't work with ByteVectorStream as reported at #796, since ByteVectorStream is not associated with a file name and FileRef detects file types based on file extensions.
This commit makes FileRef to work with ByteVectorStream by enabling it to detect file types based on the actual content of a stream.

taglib/mpc/mpcfile.cpp

@@ -75,6 +75,19 @@ public:
   Properties *properties;
 };
 
+////////////////////////////////////////////////////////////////////////////////
+// static members
+////////////////////////////////////////////////////////////////////////////////
+
+bool MPC::File::isValidStream(IOStream *stream)
+{
+  // A newer MPC file has to start with "MPCK" or "MP+", but older files don't
+  // have keys to do a quick check.
+
+  const ByteVector id = Utils::readHeader(stream, 4, false);
+  return (id == "MPCK" || id.startsWith("MP+"));
+}
+
 ////////////////////////////////////////////////////////////////////////////////
 // public members
 ////////////////////////////////////////////////////////////////////////////////

taglib/mpc/mpcfile.h

@@ -214,6 +214,15 @@ namespace TagLib {
        */
       bool hasAPETag() const;
 
+      /*!
+       * Returns whether or not the given \a stream can be opened as an MPC
+       * file.
+       *
+       * \note This method is designed to do a quick check.  The result may
+       * not necessarily be correct.
+       */
+      static bool isValidStream(IOStream *stream);
+
     private:
       File(const File &);
       File &operator=(const File &);