I think this message is a little bit confusing. You can see it when uploading a file with a filename of more than 100 characters. And it should tell you that it is talking about the file name since you are in the editor and "100 characters" could be everything there...
(For those who ask how to have filenames of >100 chars: If you have a screenshot tool for your browser which uses the title-tag as filename, for example.)
At XenDACH, where we are working on the German language pack, we found out that we would need a second phrase for that. @Alluidh said that this phrase is used every time a user entered something that is too long.
Therefore, this phrase is used (or at least has been used until 1.2.0) in the admin panel as well when you enter a node title with more than 50 characters.
So I'd say that we need a seperate phrase for a too long file name being uploaded since a file name cannot be entered.