This is source code.
You shouldn't change your model inside watch since it creates a loop. If you just need to alert when a file is uploaded whether the size is above you can just check the status.
Let's say you have this input:
Then you just need to have this in your html to show the limit size exceeded alert:
File too large
{{errorFile.size / 1000000|number:1}}MB: max 1M
And also you don't need a directive to handle that.
Check here for more info around validation.