allow downloading individual files from bundle

src/zip.rs

@@ -37,6 +37,12 @@ fn file_entries_size(files: &[UploadedFile]) -> u64 {
     total
 }
 
+pub fn file_data_offset(files: &[UploadedFile], idx: usize) -> u64 {
+    file_entries_size(&files[..idx])
+        + LOCAL_HEADER_SIZE_MINUS_FILENAME
+        + files[idx].name.len() as u64
+}
+
 fn central_directory_size(files: &[UploadedFile]) -> u64 {
     let mut total = 0;
     for file in files.iter() {