Encoding is part of the message
Base64 and quoted-printable carry content through email systems. The viewer decodes those transfer encodings before offering an attachment. A filename is sanitized for downloading, while each output receives an index to avoid collisions.
Review format limits
Ordinary attachments can be exported as their decoded bytes. Encrypted content remains encrypted. Embedded Outlook messages and unusual proprietary objects need a desktop client rather than a guessed conversion.
CSV and JSON indexes record what was read. Keep those indexes beside the EML output so you can trace a message and its attachment folder back to the selected archive.
Verify an attachment after extraction
Open a message with a known text attachment and download it from the preview. Compare its content with the original export or your mail client. The ZIP also groups attachments beneath each message’s numbered folder. Two messages can therefore both contain invoice.pdf without one replacing the other. Numbered attachment prefixes also distinguish repeated names within a message.
An attachment’s reported content type is useful metadata, not a guarantee that the file is safe to open. The direct attachment link downloads bytes as a file instead of rendering active HTML in the viewer. Inspect unfamiliar executable files or documents with your normal local security tools before opening them.
Recognize what an EML conversion changes
For EML input, the original message bytes are retained in the export while the preview uses decoded text. MSG input is reconstructed as a new MIME message using available headers, plain text and ordinary attachments. Its original Outlook properties, RTF formatting and embedded MSG objects are not a faithful round-trip target. Keep the original MSG when those details matter, and use a desktop client for unsupported object types.