Regarding the file open issue: it's not about encodings in this case but the file contains NULL bytes and Geany does not support that.
See also https://github.com/geany/geany/issues/1508 which has even more references to issues like this.
To answer the probably upcoming question when it will possible: it might be possible to implement this but it's not as trivial as it may sound and as long as nobody has a persistent interesting in developing this feature, it's unlikely it will happen.
To give something constructive: as a workaround you can replace the NULL bytes in the file and then it can be opened easily in Geany. `tr '\0' '_0_' < syslog.txt > syslog_nonull.txt`