I can successfully download a ZIP-ped file when using returning the response as application/octet-stream, but not when using multipart/form-data. The downloaded ZIP has for each file inside the ZIP exactly 2 bytes that are wrong in the corresponding file headers.

Example binary output of the ZIP files:

1. Correct ZIP content when using application/octet-stream:

PK�~�Xfirst_file.xml��,V�D���܂�Tg'��ĒDC=PK�T�CPK�~�Xsecond_file.xml��,V�D���܂�Tg'��ĒD#=PK�hPK�~�Xthird_file.xml��,V�D���܂�Tg'��ĒDc=PKW6�qPK�~�X�T�Cfirst_file.xmlPK�~�X�hYsecond_file.xmlPK�~�XW6�q�third_file.xmlPK�

2. Broken ZIP content when using multipart/form-data:

PK��Xfirst_file.xml��,V�D���܂�Tg'��ĒDC=PK�T�CPK��Xsecond_file.xml��,V�D���܂�Tg'��ĒD#=PK�hPK��Xthird_file.xml��,V�D���܂�Tg'��ĒDc=PKW6�qPK��X�T�Cfirst_file.xmlPK��X�hYsecond_file.xmlPK��XW6�q�third_file.xmlPK�

Header Differences:

The initial header bytes for each of the three files inside the ZIP are PK��, which are ZIP file signatures and headers. The bytes that differ (�~ vs �) are part of the local files’ headers in the ZIP specification, specifically bytes representing information like modification time, CRC-32, compressed size, and uncompressed size. Since these bytes are corrupted, the ZIP tool does not recognize the files properly.

The minimal code to recreate the broken version:

try {
    ResultSet rs = ...; // Here we properly set `rs`.
    MultipartFormDataOutput multipartOutput = new MultipartFormDataOutput();

    ByteArrayOutputStream zipByteOutputStream = new ByteArrayOutputStream();
    zipOutputStream = new ZipOutputStream(zipByteOutputStream);

    while (rs.next()) {
      Clob fileClob = rs.getClob("fileContent");
      ByteArrayOutputStream fileByteOutputStream = new ByteArrayOutputStream();
      // ... Here we properly fill `fileByteOutputStream`.
      
      ZipEntry entry = new ZipEntry("file.txt");
      zipOutputStream.putNextEntry(entry);
      zipOutputStream.write(fileByteOutputStream.toByteArray());
      zipOutputStream.closeEntry();
    }
    
    zipOutputStream.close();
    zipByteOutputStream.flush();

    InputStream zipInputStream =
        new ByteArrayInputStream(zipByteOutputStream.toByteArray());
    multipartOutput.addFormData(
        "file", zipInputStream, MediaType.valueOf("application/zip"), "zippedFiles.zip");

    return Response.ok(multipartOutput).build();
} catch (IOException e) {
      e.printStackTrace();
      return Response.serverError().build();
} finally {
  if (zipOutputStream != null) {
    try {
      zipOutputStream.close();
    } catch (IOException e) {
      e.printStackTrace();
    }
  }
}