diff --git a/packages/upload-client/mock-server/controllers/group.ts b/packages/upload-client/mock-server/controllers/group.ts
index 02893d120..6a4963e47 100644
--- a/packages/upload-client/mock-server/controllers/group.ts
+++ b/packages/upload-client/mock-server/controllers/group.ts
@@ -58,8 +58,8 @@ const isValidFile = (file: string): boolean => {
  * @param {object} ctx
  */
 const index = (ctx) => {
-  let files = ctx.query && ctx.query['files[]']
-  const publicKey = ctx.query && ctx.query.pub_key
+  let files = ctx.request.body && ctx.request.body['files[]']
+  const publicKey = ctx.request.body && ctx.request.body.pub_key
 
   if (!files || files.length === 0) {
     return error(ctx, {
diff --git a/packages/upload-client/mock-server/middleware/auth.ts b/packages/upload-client/mock-server/middleware/auth.ts
index 7f85b465e..df8550f7a 100644
--- a/packages/upload-client/mock-server/middleware/auth.ts
+++ b/packages/upload-client/mock-server/middleware/auth.ts
@@ -71,6 +71,11 @@ const auth = (ctx, next) => {
     publicKey: getPublicKeyFromSource(ctx.query, key)
   }
 
+  // pub_key in body
+  if (url.includes('group') && !url.includes('group/info')) {
+    params.publicKey = getPublicKeyFromSource(ctx.request.body, key)
+  }
+
   // UPLOADCARE_PUB_KEY in body
   if (
     url.includes('base') ||
diff --git a/packages/upload-client/src/api/group.ts b/packages/upload-client/src/api/group.ts
index 1209c4e7b..e2ee8388a 100644
--- a/packages/upload-client/src/api/group.ts
+++ b/packages/upload-client/src/api/group.ts
@@ -9,6 +9,7 @@ import defaultSettings from '../defaultSettings'
 import { getUserAgent } from '../tools/getUserAgent'
 import { UploadError } from '../tools/UploadError'
 import { retryIfFailed } from '../tools/retryIfFailed'
+import buildFormData from '../tools/buildFormData'
 
 export type GroupOptions = {
   publicKey: string
@@ -55,10 +56,12 @@ export default function group(
           'X-UC-User-Agent': getUserAgent({ publicKey, integration, userAgent })
         },
         url: getUrl(baseURL, '/group/', {
-          jsonerrors: 1,
-          pub_key: publicKey,
+          jsonerrors: 1
+        }),
+        data: buildFormData({
           files: uuids,
           callback: jsonpCallback,
+          pub_key: publicKey,
           signature: secureSignature,
           expire: secureExpire,
           source
diff --git a/packages/upload-client/src/tools/buildFormData.ts b/packages/upload-client/src/tools/buildFormData.ts
index 0f321b354..4c134d7f8 100644
--- a/packages/upload-client/src/tools/buildFormData.ts
+++ b/packages/upload-client/src/tools/buildFormData.ts
@@ -24,7 +24,7 @@ interface FileType extends FileOptions {
   data: SupportedFileInput
 }
 
-type InputValue = FileType | SimpleType | ObjectType
+type InputValue = FileType | SimpleType | ObjectType | SimpleType[]
 
 type FormDataOptions = {
   [key: string]: InputValue
@@ -61,7 +61,11 @@ function collectParams(
   inputKey: string,
   inputValue: InputValue
 ): void {
-  if (isFileValue(inputValue)) {
+  if (Array.isArray(inputValue)) {
+    for (const value of inputValue) {
+      collectParams(params, `${inputKey}[]`, value)
+    }
+  } else if (isFileValue(inputValue)) {
     const { name, contentType }: FileOptions = inputValue
     const file = transformFile(
       inputValue.data,
diff --git a/packages/upload-client/test/tools/buildFormData.test.ts b/packages/upload-client/test/tools/buildFormData.test.ts
index 67b54c6f3..7c9d5d558 100644
--- a/packages/upload-client/test/tools/buildFormData.test.ts
+++ b/packages/upload-client/test/tools/buildFormData.test.ts
@@ -37,6 +37,19 @@ describe('getFormDataParams', () => {
     expect(params).toContainEqual(['key', 'value'])
   })
 
+  it('should accept array parameters with simple types - string, number, undefined', () => {
+    const params = getFormDataParams({
+      key: ['string', 100, undefined]
+    })
+
+    expect(params).toEqual(
+      expect.objectContaining([
+        ['key[]', 'string'],
+        ['key[]', '100']
+      ])
+    )
+  })
+
   it('should convert numbers to strings', () => {
     const params = getFormDataParams({
       key: '1234'
@@ -66,14 +79,6 @@ describe('getFormDataParams', () => {
     expect(params.length).toEqual(0)
   })
 
-  it('should not process arrays', () => {
-    const params = getFormDataParams({
-      key1: ['1', 2, '3'] as never
-    })
-
-    expect(params.length).toEqual(0)
-  })
-
   it('should accept key-value objects, transforming any defined values to string', () => {
     const params = getFormDataParams({
       key1: {