fix(nextcloud): detect MIME type via magic bytes instead of trusting client header

Nextcloud app uploads sent application/octet-stream as Content-Type,
causing images to not be recognized. Now both WebDAV PUT and chunked
upload paths call refine_content_type() which detects via magic bytes,
then extension, then falls back to the client header. Also fixes
update_file() which previously hardcoded application/octet-stream.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

src/common/stubs.rs

@@ -467,7 +467,12 @@ impl FileUploadUseCase for StubFileUploadUseCase {
         Ok(FileDto::default())
     }
 
-    async fn update_file(&self, _path: &str, _content: &[u8]) -> Result<(), DomainError> {
+    async fn update_file(
+        &self,
+        _path: &str,
+        _content: &[u8],
+        _content_type: &str,
+    ) -> Result<(), DomainError> {
         Ok(())
     }