fix inline file preview for files with encoded URL, fix #5069 (#6525)

### Screenshot
#### Before
See #5069.
#### After
![image](/attachments/d57c2ff8-d29e-41b8-9280-c884ac43f12e)
### 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...
  - [ ] 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)).
- Manual testing: see https://codeberg.org/forgejo/forgejo/pulls/6525#issuecomment-2578814. Thanks @earl-warren.

### 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.
- [ ] 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.

Reviewed-on: https://codeberg.org/forgejo/forgejo/pulls/6525
Reviewed-by: Otto <otto@codeberg.org>
Co-authored-by: Robert Wolff <mahlzahn@posteo.de>
Co-committed-by: Robert Wolff <mahlzahn@posteo.de>

modules/markup/tests/repo/repo1_filepreview/objects/3e/2a4f1b9a15ffa15ea7ffdc06acd302442b3eca

@@ -0,0 +1 @@
+x
•ŽANÃ0EYû³GB;a	U=D9€=þ&–ÚÙÓr} 7èê­ÞÓÓëåÒŒBœ^¬´¤˜yY8Ï:AŠ†
X}<7D>R×XkÎs­"î;uìFº®9x” Œ ÊEdÐ’%Í~**Zß3\ºÙvíô9Й>nÿ8Žfxkû=<3D>[9K”%L>®ôêÙ{§<>7Ãs–;aÕvý4ÛhXOûH·Ô“þÕ†ûð`KÑ

modules/markup/tests/repo/repo1_filepreview/refs/heads/master

@@ -1 +1 @@
-c9913120ed2c1e27c1d7752ecdb7a504dc7cf6be
+eeb243c3395e1921c5d90e73bd739827251fc99d