Add comment to field fileType of LinkedFile

[InAnYan]

For https://github.com/JabRef/jabref-issue-melting-pot/issues/766.

The formulation might not be the best.

Mandatory checks

• I own the copyright of the code submitted and I licence it under the MIT license
  - [ ] Change in CHANGELOG.md described in a way that is understandable for the average user (if change is visible to the user)
  - [ ] Tests created for changes (if applicable)
• Manually tested changed features in running JabRef (always required)
  - [ ] Screenshots added in PR description (for UI changes)
• Checked developer's documentation: Is the information available and up to date? If not, I outlined it in this pull request.
• Checked documentation: Is the information available and up to date? If not, I created an issue at https://github.com/JabRef/user-documentation/issues or, even better, I submitted a pull request to the documentation repository.