FileMaker Server prints all of its logs using tab delimiters and all of its logs have the .log extension.
When selecting data files for import it is possible to select .log files using the "All Available" option. However, the import engine will often refuse to handle tab delimited log files when they are imported in this way. Sometimes it will accept them. It is may be dependent on the first few lines of the file. I've just imported one log file using this method. It refuses to import a second file. They are both Event.log files.
The standard work around is to change the .log extension to .txt. The file will then be available for selection as a tab delimited file and the imports are always successful. It would be nice to skip this step.