diff --git a/js/src/client.ts b/js/src/client.ts
index d9aa11ca4..31704d448 100644
--- a/js/src/client.ts
+++ b/js/src/client.ts
@@ -2770,6 +2770,7 @@ export class Client implements LangSmithTracingClientInterface {
         (acc, [key, value]) => {
           acc[key.slice("attachment.".length)] = {
             presigned_url: value.presigned_url,
+            mime_type: value.mime_type,
           };
           return acc;
         },
@@ -2867,6 +2868,7 @@ export class Client implements LangSmithTracingClientInterface {
             (acc, [key, value]) => {
               acc[key.slice("attachment.".length)] = {
                 presigned_url: value.presigned_url,
+                mime_type: value.mime_type || undefined,
               };
               return acc;
             },
diff --git a/js/src/schemas.ts b/js/src/schemas.ts
index 9fe4e8e16..d822e8392 100644
--- a/js/src/schemas.ts
+++ b/js/src/schemas.ts
@@ -65,6 +65,7 @@ export interface BaseExample {
 
 export interface AttachmentInfo {
   presigned_url: string;
+  mime_type?: string;
 }
 
 export type AttachmentData = Uint8Array | ArrayBuffer;
@@ -300,6 +301,7 @@ export interface Example extends BaseExample {
 interface RawAttachmentInfo {
   presigned_url: string;
   s3_url: string;
+  mime_type?: string;
 }
 export interface RawExample extends BaseExample {
   id: string;
diff --git a/js/src/tests/evaluate_attachments.int.test.ts b/js/src/tests/evaluate_attachments.int.test.ts
index 0a64a54f9..8a359e0f5 100644
--- a/js/src/tests/evaluate_attachments.int.test.ts
+++ b/js/src/tests/evaluate_attachments.int.test.ts
@@ -34,18 +34,22 @@ test("evaluate can handle examples with attachments", async () => {
     config?: TargetConfigT
   ) => {
     // Verify we receive the attachment data
-    if (!config?.attachments?.["image"]) {
+    if (!config?.attachments?.image) {
       throw new Error("Image attachment not found");
     }
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
+    const attachmentMimeType = config?.attachments?.image.mime_type;
+    if (attachmentMimeType !== "image/png") {
+      throw new Error("Image attachment has incorrect mime type");
+    }
     const attachmentData: Uint8Array | undefined = config?.attachments?.[
       "image"
     ].presigned_url
       ? new Uint8Array(
-          (await fetch(config?.attachments?.["image"].presigned_url).then(
-            (res) => res.arrayBuffer()
+          (await fetch(config?.attachments?.image.presigned_url).then((res) =>
+            res.arrayBuffer()
           )) as ArrayBuffer
         )
       : undefined;
@@ -57,14 +61,18 @@ test("evaluate can handle examples with attachments", async () => {
 
   const customEvaluator = async ({ attachments }: { attachments?: any }) => {
     expect(attachments).toBeDefined();
-    expect(attachments?.["image"]).toBeDefined();
+    expect(attachments?.image).toBeDefined();
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
-    const attachmentData: Uint8Array | undefined = attachments?.["image"]
+    const attachmentMimeType = attachments?.image.mime_type;
+    if (attachmentMimeType !== "image/png") {
+      throw new Error("Image attachment has incorrect mime type");
+    }
+    const attachmentData: Uint8Array | undefined = attachments?.image
       .presigned_url
       ? new Uint8Array(
-          (await fetch(attachments?.["image"].presigned_url).then((res) =>
+          (await fetch(attachments?.image.presigned_url).then((res) =>
             res.arrayBuffer()
           )) as ArrayBuffer
         )
@@ -134,14 +142,14 @@ test("evaluate with attachments not in target function", async () => {
 
   const customEvaluator = async ({ attachments }: { attachments?: any }) => {
     expect(attachments).toBeDefined();
-    expect(attachments?.["image"]).toBeDefined();
+    expect(attachments?.image).toBeDefined();
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
-    const attachmentData: Uint8Array | undefined = attachments?.["image"]
+    const attachmentData: Uint8Array | undefined = attachments?.image
       .presigned_url
       ? new Uint8Array(
-          (await fetch(attachments?.["image"].presigned_url).then((res) =>
+          (await fetch(attachments?.image.presigned_url).then((res) =>
             res.arrayBuffer()
           )) as ArrayBuffer
         )
@@ -210,7 +218,7 @@ test("multiple evaluators with attachments", async () => {
     config?: TargetConfigT
   ) => {
     // Verify we receive the attachment data
-    if (!config?.attachments?.["image"]) {
+    if (!config?.attachments?.image) {
       throw new Error("Image attachment not found");
     }
     const expectedData = new Uint8Array(
@@ -220,8 +228,8 @@ test("multiple evaluators with attachments", async () => {
       "image"
     ].presigned_url
       ? new Uint8Array(
-          (await fetch(config?.attachments?.["image"].presigned_url).then(
-            (res) => res.arrayBuffer()
+          (await fetch(config?.attachments?.image.presigned_url).then((res) =>
+            res.arrayBuffer()
           )) as ArrayBuffer
         )
       : undefined;
@@ -233,14 +241,14 @@ test("multiple evaluators with attachments", async () => {
 
   const customEvaluatorOne = async ({ attachments }: { attachments?: any }) => {
     expect(attachments).toBeDefined();
-    expect(attachments?.["image"]).toBeDefined();
+    expect(attachments?.image).toBeDefined();
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
-    const attachmentData: Uint8Array | undefined = attachments?.["image"]
+    const attachmentData: Uint8Array | undefined = attachments?.image
       .presigned_url
       ? new Uint8Array(
-          (await fetch(attachments?.["image"].presigned_url).then((res) =>
+          (await fetch(attachments?.image.presigned_url).then((res) =>
             res.arrayBuffer()
           )) as ArrayBuffer
         )
@@ -256,14 +264,14 @@ test("multiple evaluators with attachments", async () => {
 
   const customEvaluatorTwo = async ({ attachments }: { attachments?: any }) => {
     expect(attachments).toBeDefined();
-    expect(attachments?.["image"]).toBeDefined();
+    expect(attachments?.image).toBeDefined();
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
-    const attachmentData: Uint8Array | undefined = attachments?.["image"]
+    const attachmentData: Uint8Array | undefined = attachments?.image
       .presigned_url
       ? new Uint8Array(
-          (await fetch(attachments?.["image"].presigned_url).then((res) =>
+          (await fetch(attachments?.image.presigned_url).then((res) =>
             res.arrayBuffer()
           )) as ArrayBuffer
         )
@@ -329,7 +337,7 @@ test("evaluate with attachments runnable target function", async () => {
   await client.uploadExamplesMultipart(dataset.id, [example]);
 
   const myFunction = async (_input: any, config?: any) => {
-    if (!config?.attachments?.["image"]) {
+    if (!config?.attachments?.image) {
       throw new Error("Image attachment not found");
     }
     const expectedData = new Uint8Array(
@@ -339,8 +347,8 @@ test("evaluate with attachments runnable target function", async () => {
       "image"
     ].presigned_url
       ? new Uint8Array(
-          (await fetch(config?.attachments?.["image"].presigned_url).then(
-            (res) => res.arrayBuffer()
+          (await fetch(config?.attachments?.image.presigned_url).then((res) =>
+            res.arrayBuffer()
           )) as ArrayBuffer
         )
       : undefined;
@@ -355,14 +363,14 @@ test("evaluate with attachments runnable target function", async () => {
 
   const customEvaluator = async ({ attachments }: { attachments?: any }) => {
     expect(attachments).toBeDefined();
-    expect(attachments?.["image"]).toBeDefined();
+    expect(attachments?.image).toBeDefined();
     const expectedData = new Uint8Array(
       Buffer.from("fake image data for testing")
     );
-    const attachmentData: Uint8Array | undefined = attachments?.["image"]
+    const attachmentData: Uint8Array | undefined = attachments?.image
       .presigned_url
       ? new Uint8Array(
-          (await fetch(attachments?.["image"].presigned_url).then((res) =>
+          (await fetch(attachments?.image.presigned_url).then((res) =>
             res.arrayBuffer()
           )) as ArrayBuffer
         )