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/file_preview.go

@@ -8,6 +8,7 @@ import (
 	"bytes"
 	"html/template"
 	"io"
+	"net/url"
 	"regexp"
 	"slices"
 	"strconv"
@@ -83,6 +84,10 @@ func newFilePreview(ctx *RenderContext, node *html.Node, locale translation.Loca
 	} else if Type(filePath) != "" {
 		urlFullSource = node.Data[m[0]:m[6]] + filePath + "?display=source#" + node.Data[m[8]:m[1]]
 	}
+	filePath, err := url.QueryUnescape(filePath)
+	if err != nil {
+		return nil
+	}
 	hash := node.Data[m[8]:m[9]]
 
 	preview.start = m[0]