I have noticed this problem in several threads. The most recent example is this one.
If the orignal filename for an uploaded image was 25 characters (for example), the additional number at the end (13 characters) increases it to 38. However after a certain number of total chars, which seems to be 37 (24 + the additional 13), the string
is automatically added.
And with that neither the icon nor the actual image is displayed correctly.
So I would suggest to raise the number of allowed characters or (if not possible) at least make an announcement, that people should rename their files in advance, so that they are displayed properly.