fix(theming): fix getImage MIME type using appConfig and original file check

Signed-off-by: Anna Larch <anna@nextcloud.com>
AI-Assisted-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: miaulalala

apps/theming/lib/Controller/ThemingController.php

@@ -376,13 +376,14 @@ public function getImage(string $key, bool $useSvg = true) {
 		$csp->allowInlineStyle();
 		$response->setContentSecurityPolicy($csp);
 		$response->cacheFor(3600);
-		$response->addHeader('Content-Type', $this->config->getAppValue($this->appName, $key . 'Mime', ''));
+		// The original stored file has no extension (e.g. "logo"), so getMimeType() returns
+		// application/octet-stream for it. Use the config-stored MIME type for the original
+		// file, and getMimeType() only for converted files which have a proper extension.
+		$mimeType = $file->getName() === $key
+			? $this->appConfig->getAppValueString($key . 'Mime', '')
+			: $file->getMimeType();
+		$response->addHeader('Content-Type', $mimeType);
 		$response->addHeader('Content-Disposition', 'attachment; filename="' . $key . '"');
-		if (!$useSvg) {
-			$response->addHeader('Content-Type', 'image/png');
-		} else {
-			$response->addHeader('Content-Type', $this->config->getAppValue($this->appName, $key . 'Mime', ''));
-		}
 		return $response;
 	}