Remove seemingly needless overwrite of file bytes upon picking a file

[robholmes]

When you open a file that you don't have write permissions for, picking the file will fail, couldn't understand why as I didn't think this should be writing to the file I'm picking to read.

After digging a bit deeper I can see that upon picking a file, the code will read all file bytes and then write them to the file, effectively overwriting the original file, with the same bytes. I tried to work out if there was a reason for this behaviour, and I can't see anything, but happy to stand corrected if there's a valid reason for this.

My proposed fix for this, simply removes the calls to readAsBytes and writeAsBytes, and returns a future FileX which points to the file path, which then leaves it to the end-developer, as to whether they want to read or write to the picked file.

I've tested this on the example app, and this seems to work correctly.