This PR detects Interlisp files (files that include "(DEFINE-FILE-INFO" somewhere near the start, and do not have an .LCOM extension) as text files and displays them as such in the web UI.
To check for extensions, I had to extend the `typesniffer.DetectContentType` function to accept an extra filename parameter—which could be useful for future filetype detection features. It is possible that a few of the places I modified pass a full file path instead of just passing a file name.
Implements #8184
## Checklist
### Tests
- I added test coverage for Go changes...
- [x] in their respective `*_test.go` for unit tests.
- [ ] in the `tests/integration` directory if it involves interactions with a live Forgejo server.
- I added test coverage for JavaScript changes... - NA
- [ ] in `web_src/js/*.test.js` if it can be unit tested.
- [ ] in `tests/e2e/*.test.e2e.js` if it requires interactions with a live Forgejo server (see also the [developer guide for JavaScript testing](https://codeberg.org/forgejo/forgejo/src/branch/forgejo/tests/e2e/README.md#end-to-end-tests)).
### Documentation
- [ ] I created a pull request [to the documentation](https://codeberg.org/forgejo/docs) to explain to Forgejo users how to use this change.
- [x] I did not document these changes and I do not expect someone else to do it.
### Release notes
- [ ] I do not want this change to show in the release notes.
- [x] I want the title to show in the release notes with a link to this pull request.
- [ ] I want the content of the `release-notes/<pull request number>.md` to be be used for the release notes instead of the title.
<!--start release-notes-assistant-->
## Release notes
<!--URL:https://codeberg.org/forgejo/forgejo-->
- Features
- [PR](https://codeberg.org/forgejo/forgejo/pulls/8377): <!--number 8377 --><!--line 0 --><!--description ZGV0ZWN0IEludGVybGlzcCBzb3VyY2VzIGFzIHRleHQ=-->detect Interlisp sources as text<!--description-->
<!--end release-notes-assistant-->
Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/8377
Reviewed-by: Gusted <gusted@noreply.codeberg.org>
Co-authored-by: Bojidar Marinov <bojidar.marinov.bg@gmail.com>
Co-committed-by: Bojidar Marinov <bojidar.marinov.bg@gmail.com>
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