mod_http_upload: Suppress error on non-image files

Don't log an error (but only a debug) message if ImageMagick fails to
indentify the file type for thumbnail creation.  The image might be
encrypted, or it could be a non-image file.

Closes #809.

src/mod_http_upload.erl

@@ -856,7 +856,7 @@ identify(Path) ->
 		    height = list_to_integer(H),
 		    width = list_to_integer(W)}};
 	_ ->
-	    ?ERROR_MSG("failed to identify type of ~s: ~s", [Path, Res]),
+	    ?DEBUG("failed to identify type of ~s: ~s", [Path, Res]),
 	    {error, Res}
     end.