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Encode string attachment content to base64 in MailtrapTransport - #178
MailtrapTransport sends a string attachment content to the API as it is. The send API expects base64. Nodemailer converts a Buffer, or a string with a non-text encoding such as hex, to base64 itself. It does not change a string with no encoding or a text one (utf8, ascii, us-ascii).
The standard nodemailer example { filename: "a.txt", content: "hello" } fails with attachment 0: content should be base64 encoded. A string that is valid base64 by chance (for example "abcd") is sent with no error, and the recipient gets a corrupted file.
Changes
Encode string content to base64 as UTF-8 text, as nodemailer's SMTP path sends it
Send a string with encoding: "base64" unchanged
Add transport tests that run real nodemailer and check the API payload
Upgrade note: a string without encoding is now sent as text, the same as with nodemailer's SMTP transport. If your code passes base64 data as a string with no encoding, the recipient now gets the base64 text. Add encoding: "base64" to such attachments.
How to test
Use nodemailer.createTransport(MailtrapTransport({ token })) with a sending API token.
Send with attachments: [{ filename: "a.txt", content: "hello" }] → the send succeeds, a.txt contains hello
Send with content: "abcd" → a.txt contains abcd, not corrupted bytes
Send with content: "héllo", encoding: "us-ascii" → a.txt contains héllo
Send with content: "aGVsbG8=", encoding: "base64" → a.txt contains hello
Regression — send with content: Buffer.from("hello") and with path → both arrive unchanged
Summary by CodeRabbit
Bug Fixes
Text attachment content is now Base64-encoded before sending, including strings with UTF-8 or ASCII encoding labels.
Content already marked as Base64 remains unchanged, and binary attachments continue to be handled correctly.
Non-ASCII text attachments with a text/plain content type now use the UTF-8 charset. Other content types and ASCII text content types remain unchanged.
Nodemailer converts a Buffer attachment to base64, but sends a string
as it is. The send API expects base64, so a plain string such as
"hello" failed, and a string that was valid base64 by chance arrived as
a corrupted file.
- Decode the string with the attachment's `encoding` (UTF-8 by
default) and send it as base64.
- Send a string with `encoding: "base64"` unchanged.
- Rely on Node's Buffer for encoding names. Spellings that only
nodemailer accepts, such as "us-ascii", make sendMail reject.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Walkthrough
The attachment adapter now Base64-encodes eligible string content. For non-ASCII text/plain content, it sets the charset to UTF-8. Adapter, mail adaptation, and transport tests cover these transformations.
The adapter Base64-encodes eligible string content and updates the charset for non-ASCII text/plain content. Adapter and mail adaptation tests check the encoded content and content type.
Transport tests send attachments through Nodemailer to a mocked endpoint. They check content from strings, buffers, strings with encodings, and file paths.
CSV attachments with an explicit non-UTF-8 charset can display incorrectly. This is a bounded case to address or accept before merging.
Pre-merge checks: 4 · 1
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The title clearly and concisely describes the primary change: encoding string attachment content to base64 in MailtrapTransport.
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Inline comments:
Review comments at @src/adapters/attachement.ts:
- Line 23: In the attachment adapter, normalize the `us-ascii` encoding alias to
Node’s supported `ascii` encoding before the `Buffer.from` call, while
preserving the existing UTF-8 default and other encodings. Add a transport test
covering a string attachment with `encoding: "us-ascii"`.
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src/__tests__/adapters/mail.test.ts
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Nodemailer already decodes a string with a non-text `encoding` (such as
hex or latin1) and marks it base64. Any other string that reaches the
adapter is text, which nodemailer's own SMTP path sends as UTF-8.
- Encode such strings as UTF-8 instead of passing their `encoding` to
Buffer. This stops "us-ascii" and "utf_8" from rejecting and sends
"ascii" strings as UTF-8, as SMTP does.
- Test us-ascii, hex and `path` through the transport, and fail an
unmocked request in those tests.
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Motivation
MailtrapTransportsends a string attachmentcontentto the API as it is. The send API expects base64. Nodemailer converts a Buffer, or a string with a non-textencodingsuch ashex, to base64 itself. It does not change a string with noencodingor a text one (utf8,ascii,us-ascii).The standard nodemailer example
{ filename: "a.txt", content: "hello" }fails withattachment 0: content should be base64 encoded. A string that is valid base64 by chance (for example"abcd") is sent with no error, and the recipient gets a corrupted file.Changes
encoding: "base64"unchangedUpgrade note: a string without
encodingis now sent as text, the same as with nodemailer's SMTP transport. If your code passes base64 data as a string with noencoding, the recipient now gets the base64 text. Addencoding: "base64"to such attachments.How to test
Use
nodemailer.createTransport(MailtrapTransport({ token }))with a sending API token.attachments: [{ filename: "a.txt", content: "hello" }]→ the send succeeds,a.txtcontainshellocontent: "abcd"→a.txtcontainsabcd, not corrupted bytescontent: "héllo", encoding: "us-ascii"→a.txtcontainshéllocontent: "aGVsbG8=", encoding: "base64"→a.txtcontainshellocontent: Buffer.from("hello")and withpath→ both arrive unchangedSummary by CodeRabbit
text/plaincontent type now use the UTF-8 charset. Other content types and ASCII text content types remain unchanged.