We currently block some illegal characters but not all. This can lead to problems with uploading, renaming and generating link code.
It's possible that there is an error, but also that there's no noticable error alerting the user.
For example, the % symbol is allowed on Windows and Mac, but isn't allowed in the filename of links.
After some investigation, I came to these characters should not be allowed during upload or when renaming:
< > : " / \ | ? * \ { } ^ ` [ ] ~\
Non-printable ASCII characters (128–255 decimal characters)
ASCII character ranges 00–1F hex (0–31 decimal) and 7F (127 decimal)
%
& $ @ = ; + ,
Spaces
Multiple spaces