Fix bug #9146 (Graphics conversion problem).

Fix bug #9146 (Graphics conversion problem).

If LyX does not know about a given file format, it may easily
happen that the format is recognized as "latex" and this causes
bug #9146. This patch limits the check for a latex format to
non-binary files. The strategy for deciding that a file has
binary content is the same as that adopted by the "less" program.
This is a stripped down backport of the more complex fix in master.