diff --git a/src/__tests__/adapters/attachment.test.ts b/src/__tests__/adapters/attachment.test.ts
index aca571c3..1bdf2a25 100644
--- a/src/__tests__/adapters/attachment.test.ts
+++ b/src/__tests__/adapters/attachment.test.ts
@@ -68,7 +68,7 @@ describe("adapters/attachment: ", () => {
       });
     });
 
-    it("returns adapted attachment object.", () => {
+    it("returns adapted attachment object with string content encoded to base64.", () => {
       const attachment = {
         filename: "mock-filename",
         content: "mock-content",
@@ -76,7 +76,7 @@ describe("adapters/attachment: ", () => {
 
       const expectedAttachment = {
         filename: attachment.filename,
-        content: attachment.content,
+        content: Buffer.from(attachment.content).toString("base64"),
         disposition: undefined,
         content_id: undefined,
         type: undefined,
@@ -85,5 +85,69 @@ describe("adapters/attachment: ", () => {
 
       expect(result).toEqual(expectedAttachment);
     });
+
+    it("keeps string content that is already base64 encoded.", () => {
+      ["base64", "BASE64"].forEach((encoding) => {
+        const result = adaptAttachment({
+          filename: "mock-filename",
+          content: "abcd",
+          encoding,
+        });
+
+        expect(result.content).toBe("abcd");
+      });
+    });
+
+    it("encodes string content as UTF-8 text for text encodings.", () => {
+      [undefined, "utf8", "UTF-8", "utf_8", "ascii", "us-ascii"].forEach(
+        (encoding) => {
+          const result = adaptAttachment({
+            filename: "mock-filename",
+            content: "héllo",
+            encoding,
+          });
+
+          expect(result.content).toBe(Buffer.from("héllo").toString("base64"));
+        }
+      );
+    });
+
+    it("labels non-ASCII text/plain string content as UTF-8.", () => {
+      [
+        ["text/plain", "text/plain; charset=utf-8"],
+        ["TEXT/PLAIN", "TEXT/PLAIN; charset=utf-8"],
+        ["text/plain; charset=iso-8859-1", "text/plain; charset=utf-8"],
+        [
+          "text/plain; format=flowed",
+          "text/plain; format=flowed; charset=utf-8",
+        ],
+      ].forEach(([contentType, type]) => {
+        const result = adaptAttachment({
+          filename: "mock-filename",
+          content: "héllo",
+          contentType,
+        });
+
+        expect(result.type).toBe(type);
+      });
+    });
+
+    it("keeps the type for ASCII, base64, Buffer, or non-text/plain content.", () => {
+      [
+        { content: "hello", contentType: "text/plain" },
+        { content: "héllo", contentType: "text/csv" },
+        { content: "héllo", contentType: "text/plain-other" },
+        { content: "aMOpbGxv", contentType: "text/plain", encoding: "base64" },
+        { content: Buffer.from("héllo"), contentType: "text/plain" },
+        { content: "héllo", contentType: undefined },
+      ].forEach((attachment) => {
+        const result = adaptAttachment({
+          filename: "mock-filename",
+          ...attachment,
+        });
+
+        expect(result.type).toBe(attachment.contentType);
+      });
+    });
   });
 });
diff --git a/src/__tests__/adapters/mail.test.ts b/src/__tests__/adapters/mail.test.ts
index 5bf23315..01c35613 100644
--- a/src/__tests__/adapters/mail.test.ts
+++ b/src/__tests__/adapters/mail.test.ts
@@ -73,7 +73,12 @@ describe("adapters/mail: ", () => {
         cc: [],
         bcc: [],
         headers: data.headers,
-        attachments: data.attachments,
+        attachments: [
+          {
+            filename: "mock-filename",
+            content: Buffer.from("mock-content").toString("base64"),
+          },
+        ],
         reply_to: adaptFirstRecipient(data.replyTo),
       };
       const result = adaptMail(data);
@@ -102,7 +107,12 @@ describe("adapters/mail: ", () => {
         cc: [],
         bcc: [],
         headers: data.headers,
-        attachments: data.attachments,
+        attachments: [
+          {
+            filename: "mock-filename",
+            content: Buffer.from("mock-content").toString("base64"),
+          },
+        ],
         custom_variables: data.customVariables,
         reply_to: adaptFirstRecipient(data.replyTo),
       };
@@ -133,7 +143,7 @@ describe("adapters/mail: ", () => {
         attachments: [
           {
             filename: data.attachments[0].filename,
-            content: data.attachments[0].content,
+            content: Buffer.from("mock-content").toString("base64"),
             disposition: undefined,
             content_id: undefined,
             type: undefined,
@@ -173,7 +183,7 @@ describe("adapters/mail: ", () => {
         attachments: [
           {
             filename: data.attachments[0].filename,
-            content: data.attachments[0].content,
+            content: Buffer.from("mock-content").toString("base64"),
             disposition: undefined,
             content_id: undefined,
             type: undefined,
diff --git a/src/__tests__/lib/transport.test.ts b/src/__tests__/lib/transport.test.ts
index 4f59cd9a..fb86ce6b 100644
--- a/src/__tests__/lib/transport.test.ts
+++ b/src/__tests__/lib/transport.test.ts
@@ -1,10 +1,17 @@
+import { readFileSync } from "fs";
+
+import axios from "axios";
+import AxiosMockAdapter from "axios-mock-adapter";
+import nodemailer from "nodemailer";
+
 import mailtrapTransport from "../../lib/transport";
 
 import config from "../../config";
 
 const packageData = require("../../../package.json");
 
-const { TRANSPORT_SETTINGS } = config;
+const { CLIENT_SETTINGS, TRANSPORT_SETTINGS } = config;
+const { SENDING_ENDPOINT } = CLIENT_SETTINGS;
 const { NAME } = TRANSPORT_SETTINGS;
 
 const { version } = packageData;
@@ -79,5 +86,97 @@ describe("lib/transport: ", () => {
       // @ts-ignore
       transport.send(message, callback);
     });
+
+    describe("attachments: ", () => {
+      let mock: AxiosMockAdapter;
+
+      beforeAll(() => {
+        mock = new AxiosMockAdapter(axios);
+        mock
+          .onPost(`${SENDING_ENDPOINT}/api/send`)
+          .reply(200, { success: true, message_ids: ["mock-message-id"] })
+          .onAny()
+          .reply(404);
+      });
+
+      afterEach(() => {
+        mock.resetHistory();
+      });
+
+      afterAll(() => {
+        mock.restore();
+      });
+
+      const sendAttachment = async (attachment: object) => {
+        const transport = nodemailer.createTransport(
+          mailtrapTransport({ token: "mock-token" })
+        );
+
+        await transport.sendMail({
+          from: "sender@example.com",
+          to: "recipient@example.com",
+          subject: "mock-subject",
+          text: "mock-text",
+          attachments: [{ filename: "mock.txt", ...attachment }],
+        });
+
+        const [request] = mock.history.post;
+
+        return JSON.parse(request.data).attachments[0];
+      };
+
+      it("sends string content encoded to base64.", async () => {
+        const { content } = await sendAttachment({ content: "hello" });
+
+        expect(content).toBe(Buffer.from("hello").toString("base64"));
+      });
+
+      it("sends string content with a text encoding as UTF-8.", async () => {
+        const { content } = await sendAttachment({
+          content: "héllo",
+          encoding: "us-ascii",
+        });
+
+        expect(content).toBe(Buffer.from("héllo").toString("base64"));
+      });
+
+      it("labels non-ASCII string content with the UTF-8 charset.", async () => {
+        const { type } = await sendAttachment({ content: "héllo" });
+
+        expect(type).toBe("text/plain; charset=utf-8");
+      });
+
+      it("sends string content with `base64` encoding as it is.", async () => {
+        const { content } = await sendAttachment({
+          content: "aGVsbG8=",
+          encoding: "base64",
+        });
+
+        expect(content).toBe("aGVsbG8=");
+      });
+
+      it("sends string content with another encoding decoded.", async () => {
+        const { content } = await sendAttachment({
+          content: "68656c6c6f",
+          encoding: "hex",
+        });
+
+        expect(content).toBe(Buffer.from("hello").toString("base64"));
+      });
+
+      it("sends Buffer content encoded to base64.", async () => {
+        const { content } = await sendAttachment({
+          content: Buffer.from("hello"),
+        });
+
+        expect(content).toBe(Buffer.from("hello").toString("base64"));
+      });
+
+      it("sends file content from `path` encoded to base64.", async () => {
+        const { content } = await sendAttachment({ path: __filename });
+
+        expect(content).toBe(readFileSync(__filename).toString("base64"));
+      });
+    });
   });
 });
diff --git a/src/adapters/attachement.ts b/src/adapters/attachement.ts
index d7ccfc64..ccb9849c 100644
--- a/src/adapters/attachement.ts
+++ b/src/adapters/attachement.ts
@@ -8,6 +8,48 @@ import { NodemailerAttachment } from "../types/transport";
 const { ERRORS } = CONFIG;
 const { FILENAME_REQUIRED } = ERRORS;
 
+function isUtf8Text(
+  content: string | Buffer,
+  encoding: string | undefined
+): content is string {
+  return typeof content === "string" && encoding?.toLowerCase() !== "base64";
+}
+
+/**
+ * Returns string content in base64, as the Mailtrap API takes it.
+ * Nodemailer decodes a string with a non-text `encoding` and marks it `base64`, so any other string is text, which it sends as UTF-8.
+ */
+function encodeContent(
+  content: string | Buffer,
+  encoding: string | undefined
+): string | Buffer {
+  if (!isUtf8Text(content, encoding)) {
+    return content;
+  }
+
+  return Buffer.from(content, "utf8").toString("base64");
+}
+
+/**
+ * Labels non-ASCII text/plain as UTF-8, the bytes encodeContent produces, as Nodemailer's SMTP path does.
+ */
+function adaptType(
+  type: string | undefined,
+  content: string | Buffer,
+  encoding: string | undefined
+): string | undefined {
+  if (
+    !type ||
+    !/^text\/plain\s*(;|$)/i.test(type) ||
+    !isUtf8Text(content, encoding) ||
+    !/[\u0080-￿]/.test(content)
+  ) {
+    return type;
+  }
+
+  return `${type.replace(/\s*;\s*charset=[^;]*/i, "")}; charset=utf-8`;
+}
+
 /**
  * Adopts Nodemailer attachment to Mailtrap.
  * Checks if filename or content are missing, then rejects with error.
@@ -21,11 +63,14 @@ export default function adaptAttachment(
     throw new Error(FILENAME_REQUIRED);
   }
 
+  const content = adaptContent(nodemailerAttachment.content);
+  const { encoding } = nodemailerAttachment;
+
   return {
     filename: nodemailerAttachment.filename,
-    content: adaptContent(nodemailerAttachment.content),
+    content: encodeContent(content, encoding),
     disposition: nodemailerAttachment.contentDisposition,
     content_id: nodemailerAttachment.cid,
-    type: nodemailerAttachment.contentType,
+    type: adaptType(nodemailerAttachment.contentType, content, encoding),
   };
 }