A file type will expect and demand that the data in it is in a particular form - it will have check words to guarantee that it has not been corrupted.
If you try to change the file with the wrong type of data or without recomputing the check words the file's integrity will be compromised - violated. The machine tries hard to stop you from doing this.
You will need to find another way to make the changes - the file may be write protected.
Mostly guesswork... But the question might have been clearer.
Peccavi, March 2011
