Preview does not display anything for files with ":" in filename

- [x] I have fzf 0.23.0 or above
- [x] I have read through https://github.com/junegunn/fzf.vim/blob/master/README.md
- [x] I have read through https://github.com/junegunn/fzf/blob/master/README-VIM.md
- [x] I have read through the manual page of fzf (`man fzf`)
- [x] I have searched through the existing issues

# How to reproduce:

```
user@dev:~/test-rg$ echo "Bye" > a:b && echo "Hello" > a
user@dev:~/test-rg$ nvim -Nu <(curl https://gist.githubusercontent.com/junegunn/6936bf79fedd3a079aeb1dd2f3c81ef5/raw)
```
Type `:Files` or `:Rg` and select the file named "a:b".

# Expected Behavior:

Preview window will show content of file "a:b".

# Actual Behavior:

No preview is shown for "a:b", or any file with ":" in its filename.

# Steps I tried to resolve the behavior:

Setting "isfname+=:" does not help.