I found this fix on a Skype community thread:
Quit Skype, then in your home directory, look for a folder called Library/Caches/com.skype.skype and delete or rename that folder then start Skype... cpu usage should be back to normal.
If you can't find that folder, open your Terminal and simply type:
$ open ~/Library/Caches/com.skype.skype
I think I've got an answer.
Googling around I've found references to "grayed out" problems and mentions of the file's creator attribute.
So a quick Google for "osx file creator mark" pointed me to SetFileand its sibling GetFileInfo.
Running a quick GetFileInfo on the files I get:
Mac:samples jjarava$ for i in 201507*.TXT; do getfileinfo "$i"; echo .; done
file: "/path/to/samples/20150728 - Source Fragment 3 (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Framgent 1 (Good).TXT"
type: "TEXT"
creator: "\0\0\0\0"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Framgent 2 (Good).TXT"
type: "TEXT"
creator: "\0\0\0\0"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
file: "/path/to/samples/20150728 - Source Fragment 3 copy (BAD).TXT"
type: "????"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
.
All the "working" files are of type: "TEXT", and all the "not working" ones seem not to have a "type" defined...
Actually, running the following to change the file type:
Mac:samples jjarava$ setfile -t TEXT "20150728 - Source Fragment 3 copy (BAD).TXT"
Mac:samples jjarava$ getfileinfo "20150728 - Source Fragment 3 copy (BAD).TXT"
file: "/path/to/samples/20150728 - Source Fragment 3 copy (BAD).TXT"
type: "TEXT"
creator: "????"
attributes: avbstclinmedz
created: 08/03/2015 13:58:18
modified: 08/03/2015 13:58:18
And that file can now be selected in the "Open" dialog in Excel!!
The question is where does the "type" field come from, and why it's set in some files and not in others, but at least there's some "logic" to the issue!!
Best Answer
Have you actually tried?
As I have used CommandZ multiple times on formulae I have edited several times and it steps back through the changes sequentially. Even changes sheets as necessary.
On version 16.29 & 365, the help file says 100 actions can be undone but there are exceptions...
Just tested by entering 0 to 9 in separate cells - CommandZ goes back through deleting them in the order they were entered.