diff --git a/package-lock.json b/package-lock.json
index fdac779f3..eed23b554 100644
--- a/package-lock.json
+++ b/package-lock.json
@@ -11,9 +11,11 @@
       "dependencies": {
         "axios": "^1.7.9",
         "formdata-node": "^6.0.3",
+        "mime-types": "^2.1.35",
         "tslib": "^2.8.1"
       },
       "devDependencies": {
+        "@types/mime-types": "^2.1.4",
         "@types/node": "^18.19.69",
         "@types/sinon": "^17.0.3",
         "@typescript-eslint/eslint-plugin": "^8.19.0",
@@ -1048,6 +1050,13 @@
         "@types/unist": "*"
       }
     },
+    "node_modules/@types/mime-types": {
+      "version": "2.1.4",
+      "resolved": "https://registry.npmjs.org/@types/mime-types/-/mime-types-2.1.4.tgz",
+      "integrity": "sha512-lfU4b34HOri+kAY5UheuFMWPDOI+OPceBSHZKp69gEyTL/mmJ4cnU6Y/rlme3UL3GyOn6Y42hyIEw0/q8sWx5w==",
+      "dev": true,
+      "license": "MIT"
+    },
     "node_modules/@types/ms": {
       "version": "0.7.34",
       "resolved": "https://registry.npmjs.org/@types/ms/-/ms-0.7.34.tgz",
diff --git a/package.json b/package.json
index b9d82215a..fe3e11941 100644
--- a/package.json
+++ b/package.json
@@ -53,6 +53,7 @@
     "code:formatting": "npm run replace:all && npm run prettier && npm run lint:fix"
   },
   "devDependencies": {
+    "@types/mime-types": "^2.1.4",
     "@types/node": "^18.19.69",
     "@types/sinon": "^17.0.3",
     "@typescript-eslint/eslint-plugin": "^8.19.0",
@@ -73,6 +74,7 @@
   "dependencies": {
     "axios": "^1.7.9",
     "formdata-node": "^6.0.3",
+    "mime-types": "^2.1.35",
     "tslib": "^2.8.1"
   }
 }
diff --git a/src/serviceDesk/parameters/attachTemporaryFile.ts b/src/serviceDesk/parameters/attachTemporaryFile.ts
index f1ea8ab2b..ed93037e0 100644
--- a/src/serviceDesk/parameters/attachTemporaryFile.ts
+++ b/src/serviceDesk/parameters/attachTemporaryFile.ts
@@ -1,6 +1,7 @@
 export interface Attachment {
   filename: string;
   file: Buffer | ReadableStream | string | Blob | File;
+  mimeType?: string;
 }
 
 export interface AttachTemporaryFile {
diff --git a/src/serviceDesk/serviceDesk.ts b/src/serviceDesk/serviceDesk.ts
index ec3d824f1..99123a87e 100644
--- a/src/serviceDesk/serviceDesk.ts
+++ b/src/serviceDesk/serviceDesk.ts
@@ -1,4 +1,5 @@
 import { FormData } from 'formdata-node';
+import * as mime from 'mime-types';
 import * as Models from './models';
 import * as Parameters from './parameters';
 import { Callback } from '../callback';
@@ -116,7 +117,14 @@ export class ServiceDesk {
     const formData = new FormData();
     const attachments = Array.isArray(parameters.attachment) ? parameters.attachment : [parameters.attachment];
 
-    attachments.forEach(attachment => formData.append('file', attachment.file, attachment.filename));
+    attachments.forEach(attachment => {
+      const mimeType = attachment.mimeType ?? (mime.lookup(attachment.filename) || undefined);
+      const file = Buffer.isBuffer(attachment.file)
+        ? new File([attachment.file], attachment.filename, { type: mimeType })
+        : attachment.file;
+
+      formData.append('file', file, attachment.filename);
+    });
 
     const config: RequestConfig = {
       url: `/rest/servicedeskapi/servicedesk/${parameters.serviceDeskId}/attachTemporaryFile`,
diff --git a/src/version2/issueAttachments.ts b/src/version2/issueAttachments.ts
index 33bad5b82..29dc8fb56 100644
--- a/src/version2/issueAttachments.ts
+++ b/src/version2/issueAttachments.ts
@@ -1,4 +1,5 @@
 import { FormData } from 'formdata-node';
+import * as mime from 'mime-types';
 import * as Models from './models';
 import * as Parameters from './parameters';
 import { Callback } from '../callback';
@@ -425,7 +426,14 @@ export class IssueAttachments {
     const formData = new FormData();
     const attachments = Array.isArray(parameters.attachment) ? parameters.attachment : [parameters.attachment];
 
-    attachments.forEach(attachment => formData.append('file', attachment.file, attachment.filename));
+    attachments.forEach(attachment => {
+      const mimeType = attachment.mimeType ?? (mime.lookup(attachment.filename) || undefined);
+      const file = Buffer.isBuffer(attachment.file)
+        ? new File([attachment.file], attachment.filename, { type: mimeType })
+        : attachment.file;
+
+      formData.append('file', file, attachment.filename);
+    });
 
     const config: RequestConfig = {
       url: `/rest/api/2/issue/${parameters.issueIdOrKey}/attachments`,
diff --git a/src/version2/parameters/addAttachment.ts b/src/version2/parameters/addAttachment.ts
index aff6535be..252176395 100644
--- a/src/version2/parameters/addAttachment.ts
+++ b/src/version2/parameters/addAttachment.ts
@@ -1,6 +1,7 @@
 export interface Attachment {
   filename: string;
   file: Buffer | ReadableStream | string | Blob | File;
+  mimeType?: string;
 }
 
 export interface AddAttachment {
diff --git a/src/version3/issueAttachments.ts b/src/version3/issueAttachments.ts
index b134f7a4f..3eaa7b45d 100644
--- a/src/version3/issueAttachments.ts
+++ b/src/version3/issueAttachments.ts
@@ -1,4 +1,5 @@
 import { FormData } from 'formdata-node';
+import * as mime from 'mime-types';
 import * as Models from './models';
 import * as Parameters from './parameters';
 import { Callback } from '../callback';
@@ -425,7 +426,14 @@ export class IssueAttachments {
     const formData = new FormData();
     const attachments = Array.isArray(parameters.attachment) ? parameters.attachment : [parameters.attachment];
 
-    attachments.forEach(attachment => formData.append('file', attachment.file, attachment.filename));
+    attachments.forEach(attachment => {
+      const mimeType = attachment.mimeType ?? (mime.lookup(attachment.filename) || undefined);
+      const file = Buffer.isBuffer(attachment.file)
+        ? new File([attachment.file], attachment.filename, { type: mimeType })
+        : attachment.file;
+
+      formData.append('file', file, attachment.filename);
+    });
 
     const config: RequestConfig = {
       url: `/rest/api/3/issue/${parameters.issueIdOrKey}/attachments`,
diff --git a/src/version3/parameters/addAttachment.ts b/src/version3/parameters/addAttachment.ts
index aff6535be..252176395 100644
--- a/src/version3/parameters/addAttachment.ts
+++ b/src/version3/parameters/addAttachment.ts
@@ -1,6 +1,7 @@
 export interface Attachment {
   filename: string;
   file: Buffer | ReadableStream | string | Blob | File;
+  mimeType?: string;
 }
 
 export interface AddAttachment {