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Copying file has unexpected side effect

I have a laptop with multiple installs of Debian Jessie using MATE desktop. There are minor differences of package complements - the purpose being to determine an optimal configuration.

There are a relatively small number of files which I would like to have the latest version available no matter which install is active.

My solution was to place this files on a separate partition of the hdd.
It will be mounted at boot. The fstab entry is currently
UUID=E90C-65B4  /media/common vfat auto,exec,rw,flush,umask=000  0 0

The problem occurred on the very first use:
I opened /media/common by double-clicking its desktop icon.
I then:
  right-clicked on the desktop icon of a text file
  selected "Copy" from the menu
  moved mouse over the displayed directory of /media/common
  right-clicked and chose "Paste" from menu

The file was _apparently_ copied as expected.
*HOWEVER* the act of copying set the execution flag.