code: @Override
      public Set<String> getFileExtension(String fileTypeExtension) throws OSException {
          fileTypeExtension = sanitizeFileExtension(fileTypeExtension);
  
          if (!LinuxMIMEDatabase.instance().getExtensionMimeMap().containsKey(fileTypeExtension)) {
              throw new OSException("File type extension \"" + fileTypeExtension + "\" is not registered.");
          }
  
          Set<String> apps = new HashSet<>();
          Set<String> allMimeTypes = LinuxMIMEDatabase.instance().getExtensionMimeMap().get(fileTypeExtension);
          Map<String, Set<String>> mimeApplications = LinuxMIMEDatabase.instance().getMimeApps();
          for (String mimeType : allMimeTypes) {
              if (mimeApplications.containsKey(mimeType) && mimeApplications.get(mimeType) != null) {
                  apps.addAll(mimeApplications.get(mimeType));
              }
          }
          return apps;
      }
docs: Returns the associated application for a given extension.
  
  @param fileTypeExtension File extension with leading dot, e.g.
  <code>.bar</code>.
  @return {@link String} with path to associated application or
  <code>null</code> if file extension is not registered or can't be read.
  @throws OSException