fix(preview): Filter for folders in cleanup old preview job

Fixes #35936.

When running `OC\Preview\BackgroundCleanupJob`, the main iteration loop

in `run()` expects a folder, however, `getOldPreviewLocations()`

currently does not filter by mimetype and therefore can yield a

non-folder entry which causes an Exception when constructing the Folder

impl.

Filtering for `httpd/unix-directory`, as `getNewPreviewLocations()`

already does, fixes this issue.

Signed-off-by: Dario Mehlich <d.mehlich@gmail.com>

[skip ci]

Author: hammer065

tests/lib/Preview/BackgroundCleanupJobTest.php

@@ -197,13 +197,33 @@ public function testOldPreviews() {
 		$f2 = $appdata->newFolder('123456782');
 		$f2->newFile('foo.jpg', 'foo');
 
+		/*
+		 * Cleanup of OldPreviewLocations should only remove numeric folders on AppData level,
+		 * therefore these files should stay untouched.
+		 */
+		$appdata->getFolder('/')->newFile('not-a-directory', 'foo');
+		$appdata->getFolder('/')->newFile('133742', 'bar');
+
 		$appdata = \OC::$server->getAppDataDir('preview');
 		$this->assertSame(2, count($appdata->getDirectoryListing()));
 
 		$job = new BackgroundCleanupJob($this->timeFactory, $this->connection, $this->getRoot(), $this->mimeTypeLoader, true);
 		$job->run([]);
 
 		$appdata = \OC::$server->getAppDataDir('preview');
+
+		// Check if the files created above are still present
+		// Remember: AppData::getDirectoryListing filters all non-folders
 		$this->assertSame(0, count($appdata->getDirectoryListing()));
+		try {
+			$appdata->getFolder('/')->getFile('not-a-directory');
+		} catch (NotFoundException) {
+			$this->fail('Could not find file \'not-a-directory\'');
+		}
+		try {
+			$appdata->getFolder('/')->getFile('133742');
+		} catch (NotFoundException) {
+			$this->fail('Could not find file \'133742\'');
+		}
 	}
 }