Every email attachment you send is Base64-encoded using MIME (Multipurpose Internet Mail Extensions). This tool helps you encode files for raw email construction or decode attachment data for debugging delivery issues.
Email developers building SMTP integrations, debugging failed attachments, or constructing raw MIME messages need to Base64 encode/decode attachment data manually.
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How to Base64 for Email Attachments — MIME Encoding
- Upload the file you want to attach (or paste encoded data)
- For encoding: the tool converts the file to MIME-compatible Base64
- For decoding: paste the Base64 block from raw email source
- Copy the encoded string for your MIME message
- Add Content-Transfer-Encoding: base64 header in your email
Pro Tips
- MIME Base64 wraps lines at 76 characters — standard email format
- Content-Type header must match the file: application/pdf, image/png, etc.
- Maximum email size varies by provider: Gmail 25MB, Outlook 20MB, Yahoo 25MB
- Base64 adds 33% size — a 15MB file becomes 20MB encoded (may exceed limits)
Frequently Asked Questions
Why do emails use Base64?
Email (SMTP) is a text-only protocol from the 1970s. It cannot transmit binary data directly. Base64 converts binary attachments to text that SMTP can handle safely.
How do I decode an email attachment manually?
View the raw email source, find the Base64 block after Content-Transfer-Encoding: base64, copy it, and decode with this tool. Save the output as the original file type.
Why is my email attachment corrupted?
Common causes: truncated Base64 (missing padding), wrong Content-Type header, or line breaks in wrong places. Decode and re-encode to fix.
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