KeyWord here is:
Exception type: NullReferenceException
Exception message: Object reference not set to an instance of an object.
Sounds like when you are attaching for some reason its not picking up the file so its coming up as "Blank"
causing a "Null" error because to the server you are trying to upload nothing.
Now the question why is it not picking up the file?
Did you try different files and images to upload?
Did you try the "Page" editor too?
Maybe the upload process is not working.
Did you check the /media folder to see if the file you are uploading
is on your server?
Or check the /files folder
If the files and images are on your server then thats not the issue
but if its not then its an upload issue.
1. Uploading to server
2. Attaching the files to the editor
3. Rights issue, (Rights not set correctly for the app_data folder, files have "read only", and etc.)
I think thats it.
So you need to go down this list in order till you figure it out :)
Have a great day !!