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/application/services/file_upload_service.rs

@@ -222,7 +222,12 @@ impl FileUploadUseCase for FileUploadService {
     ///
     /// Spools the in-memory `&[u8]` to a temp file with hash-on-write,
     /// then delegates to the streaming update/create path.
-    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> {
         // Spool to temp file + hash
         let temp = tempfile::NamedTempFile::new()
             .map_err(|e| DomainError::internal_error("FileUpload", format!("temp file: {e}")))?;
@@ -235,7 +240,7 @@ impl FileUploadUseCase for FileUploadService {
             path,
             temp.path(),
             content.len() as u64,
-            "application/octet-stream",
+            content_type,
             Some(hash),
         )
         .await