Bugfix #4649

Three issues:
- When saving a file, without extension added, and no filename provided,
  the saving code received the directory name only. That's a potential
  danger of getting directories deleted.
  Added in the saveover() function a check for this, and return an error
  when you try to save over a directory.

- Screendump did not add file extensions yet, when indicated todo so.

- Screendump code was duplicating all image type cases, whilst we have a
  nice BKE_write_ibuf() call for that now. (Bug was that this code did not
  check for BMP, saving the file in default format.)

source/blender/src/toolbox.c

@@ -232,7 +232,20 @@ void error(char *fmt, ...)
 
 int saveover(char *file)
 {
-	return (!BLI_exists(file) || confirm("Save over", file));
+	int len= strlen(file);
+	
+	if(len==0) 
+		return 0;
+	
+	if(BLI_exists(file)==0)
+		return 1;
+	
+	if( file[len-1]=='/' || file[len-1]=='\\' ) {
+		error("Cannot overwrite a directory");
+		return 0;
+	}
+		
+	return confirm("Save over", file);
 }
 
 /* ****************** EXTRA STUFF **************** */