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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
Attachments containing text are now Base64-encoded before they are sent, including content with UTF-8 or ASCII encoding labels.
Content already marked as Base64 remains unchanged, and binary attachments continue to be handled correctly across supported attachment sources.
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>
No actionable comments were generated in the recent review.
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src/__tests__/adapters/attachment.test.ts
src/__tests__/lib/transport.test.ts
src/adapters/attachement.ts
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Walkthrough
The attachment adapter now Base64-encodes string content unless it is already marked as Base64. Adapter, mail adaptation, and transport tests cover encoded content for multiple attachment input types.
Transport tests send attachments through Nodemailer to a mocked endpoint. They check content from strings, buffers, strings with encodings, and file paths.
String attachments are encoded after Nodemailer normalization. No material new merge risk was established.
Pre-merge checks: 5
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The title clearly and concisely identifies the main change: encoding string attachment content to base64 in MailtrapTransport.
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Actionable comments posted: 1
🪄 Fix comments: Fix CodeRabbit comments on this PR
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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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Files selected for processing: 4
src/__tests__/adapters/attachment.test.ts
src/__tests__/adapters/mail.test.ts
src/__tests__/lib/transport.test.ts
src/adapters/attachement.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