diff --git a/package.json b/package.json
index d118e061c..aa3129e5f 100644
--- a/package.json
+++ b/package.json
@@ -43,7 +43,7 @@
     "exenv": "^1.2.1",
     "express": "^4.13.4",
     "file-saver": "^1.3.3",
-    "flat": "^2.0.1",
+    "flat": "^4.0.0",
     "flow-bin": "^0.46.0",
     "global": "^4.3.1",
     "immutable": "^3.8.1",
diff --git a/src/redux/helpers/api/tools.js b/src/redux/helpers/api/tools.js
index ab71dc6e8..6d444333d 100644
--- a/src/redux/helpers/api/tools.js
+++ b/src/redux/helpers/api/tools.js
@@ -1,6 +1,6 @@
 import statusCode from 'statuscode';
 import { addNotification } from '../../modules/notifications';
-import flatten from 'flat';
+import { flatten } from 'flat';
 
 import { logout } from '../../modules/auth';
 import { isTokenValid, decode } from '../../helpers/token';
@@ -14,6 +14,17 @@ export const API_BASE = process.env.API_BASE || 'http://localhost:4000/v1';
 const maybeShash = endpoint => (endpoint.indexOf('/') === 0 ? '' : '/');
 const getUrl = endpoint => API_BASE + maybeShash(endpoint) + endpoint;
 
+const maybeQuestionMark = (endpoint, query) =>
+  Object.keys(query).length === 0
+    ? ''
+    : endpoint.indexOf('?') === -1 ? '?' : '&';
+
+const generateQuery = query =>
+  !query ? '' : Object.keys(query).map(key => `${key}=${query[key]}`).join('&');
+
+export const assembleEndpoint = (endpoint, query = {}) =>
+  endpoint + maybeQuestionMark(endpoint, query) + generateQuery(query);
+
 export const flattenBody = body => {
   const flattened = flatten(body, { delimiter: ':' });
   body = {};
@@ -36,33 +47,45 @@ const createFormData = body => {
   return data;
 };
 
-const maybeQuestionMark = (endpoint, query) =>
-  Object.keys(query).length === 0
-    ? ''
-    : endpoint.indexOf('?') === -1 ? '?' : '&';
-
-const generateQuery = query =>
-  !query ? '' : Object.keys(query).map(key => `${key}=${query[key]}`).join('&');
+const encodeBody = (body, method, encodeAsMultipart) => {
+  if (method.toUpperCase() !== 'POST') {
+    return undefined;
+  }
 
-export const assembleEndpoint = (endpoint, query = {}) =>
-  endpoint + maybeQuestionMark(endpoint, query) + generateQuery(query);
+  if (encodeAsMultipart) {
+    return body ? createFormData(body) : undefined;
+  } else {
+    // otherwise we encode in JSON
+    return JSON.stringify(body || []);
+  }
+};
 
-export const createRequest = (endpoint, query = {}, method, headers, body) =>
+export const createRequest = (
+  endpoint,
+  query = {},
+  method,
+  headers,
+  body,
+  uploadFiles
+) =>
   fetch(getUrl(assembleEndpoint(endpoint, query)), {
     method,
     headers,
-    body: body ? createFormData(body) : undefined
+    body: encodeBody(body, method, uploadFiles)
   });
 
-export const getHeaders = (headers, accessToken) => {
+export const getHeaders = (headers, accessToken, skipContentType) => {
+  const usedHeaders = skipContentType
+    ? headers
+    : { 'Content-Type': 'application/json', ...headers };
   if (accessToken) {
     return {
       Authorization: `Bearer ${accessToken}`,
-      ...headers
+      ...usedHeaders
     };
   }
 
-  return headers;
+  return usedHeaders;
 };
 
 /**
@@ -79,11 +102,12 @@ export const createApiCallPromise = (
     accessToken = '',
     body = undefined,
     wasSuccessful = () => true,
-    doNotProcess = false
+    doNotProcess = false,
+    uploadFiles = false
   },
   dispatch = undefined
 ) => {
-  let call = createRequest(endpoint, query, method, headers, body)
+  let call = createRequest(endpoint, query, method, headers, body, uploadFiles)
     .catch(err => detectUnreachableServer(err, dispatch))
     .then(res => {
       if (
diff --git a/src/redux/middleware/apiMiddleware.js b/src/redux/middleware/apiMiddleware.js
index 7ba01ac64..2bb8c3a9d 100644
--- a/src/redux/middleware/apiMiddleware.js
+++ b/src/redux/middleware/apiMiddleware.js
@@ -18,7 +18,8 @@ export const apiCall = (
     body = undefined,
     meta = undefined,
     wasSuccessful = isTwoHundredCode,
-    doNotProcess = false
+    doNotProcess = false,
+    uploadFiles = false
   },
   dispatch = undefined
 ) => ({
@@ -29,10 +30,11 @@ export const apiCall = (
         endpoint,
         query,
         method,
-        headers: getHeaders(headers, accessToken),
+        headers: getHeaders(headers, accessToken, uploadFiles),
         body,
         wasSuccessful,
-        doNotProcess
+        doNotProcess,
+        uploadFiles
       },
       dispatch
     ),
diff --git a/src/redux/modules/additionalExerciseFiles.js b/src/redux/modules/additionalExerciseFiles.js
index 05353554c..ddbfcfe52 100644
--- a/src/redux/modules/additionalExerciseFiles.js
+++ b/src/redux/modules/additionalExerciseFiles.js
@@ -41,7 +41,8 @@ export const addAdditionalExerciseFiles = (exerciseId, files) =>
         tmpId: Math.random().toString(),
         file: uploaded.file
       }))
-    }
+    },
+    uploadFiles: true
   });
 
 export const removeAdditionalExerciseFile = (exerciseId, fileId) =>
diff --git a/src/redux/modules/pipelineFiles.js b/src/redux/modules/pipelineFiles.js
index 6da682957..a4775437d 100644
--- a/src/redux/modules/pipelineFiles.js
+++ b/src/redux/modules/pipelineFiles.js
@@ -39,7 +39,8 @@ export const addPipelineFiles = (pipelineId, files) =>
         tmpId: Math.random().toString(),
         file: uploaded.file
       }))
-    }
+    },
+    uploadFiles: true
   });
 
 /**
diff --git a/src/redux/modules/supplementaryFiles.js b/src/redux/modules/supplementaryFiles.js
index c6b20f6af..361a4566d 100644
--- a/src/redux/modules/supplementaryFiles.js
+++ b/src/redux/modules/supplementaryFiles.js
@@ -41,7 +41,8 @@ export const addSupplementaryFiles = (exerciseId, files) =>
         tmpId: Math.random().toString(),
         file: uploaded.file
       }))
-    }
+    },
+    uploadFiles: true
   });
 
 export const removeSupplementaryFile = (exerciseId, fileId) =>
diff --git a/src/redux/modules/upload.js b/src/redux/modules/upload.js
index f84603e7e..df6858788 100644
--- a/src/redux/modules/upload.js
+++ b/src/redux/modules/upload.js
@@ -21,7 +21,8 @@ export const uploadFile = (id, file, endpoint = '/uploaded-files') =>
     method: 'POST',
     endpoint,
     body: { [file.name]: file },
-    meta: { id, fileName: file.name }
+    meta: { id, fileName: file.name },
+    uploadFiles: true
   });
 
 const wrapWithName = (id, file) => ({ [file.name]: file });
diff --git a/test/redux/helpers/api/tools-test.js b/test/redux/helpers/api/tools-test.js
index 5a5a19b7d..3701429ce 100644
--- a/test/redux/helpers/api/tools-test.js
+++ b/test/redux/helpers/api/tools-test.js
@@ -2,8 +2,7 @@ import { expect } from 'chai';
 
 import {
   getHeaders,
-  assembleEndpoint,
-  flattenBody
+  assembleEndpoint
 } from '../../../../src/redux/helpers/api/tools';
 
 describe('API middleware and helper functions', () => {
@@ -11,7 +10,8 @@ describe('API middleware and helper functions', () => {
     it('must add access token to the headers', () => {
       expect(getHeaders({ a: 'b' }, 'abcd')).to.eql({
         a: 'b',
-        Authorization: 'Bearer abcd'
+        Authorization: 'Bearer abcd',
+        'Content-Type': 'application/json'
       });
     });
 
@@ -33,36 +33,5 @@ describe('API middleware and helper functions', () => {
         'http://www.blabla.com/abcd'
       );
     });
-
-    it('must flatten the body object to urlencoded format', () => {
-      expect(flattenBody({})).to.eql({});
-      expect(flattenBody([])).to.eql({});
-      expect(flattenBody({ a: 'b' })).to.eql({ a: 'b' });
-      expect(flattenBody({ a: { b: 'c' } })).to.eql({ 'a[b]': 'c' });
-      expect(flattenBody({ a: { b: { c: 'd' } } })).to.eql({ 'a[b][c]': 'd' });
-      expect(flattenBody({ a: { b: ['c', 'd'] } })).to.eql({
-        'a[b][0]': 'c',
-        'a[b][1]': 'd'
-      });
-      expect(
-        flattenBody({
-          a: [
-            {
-              b: [{ c: 'A' }, { d: 'B' }]
-            },
-            [{ f: 'C', g: 'D' }, { h: 'E' }]
-          ]
-        })
-      ).to.eql({
-        'a[0][b][0][c]': 'A',
-        'a[0][b][1][d]': 'B',
-        'a[1][0][f]': 'C',
-        'a[1][0][g]': 'D',
-        'a[1][1][h]': 'E'
-      });
-
-      // not a POJO
-      expect(() => flattenBody({ a: new MouseEvent('click') })).to.throw();
-    });
   });
 });
diff --git a/yarn.lock b/yarn.lock
index d1d3fec97..8a39bc5fc 100644
--- a/yarn.lock
+++ b/yarn.lock
@@ -2991,11 +2991,11 @@ flat-cache@^1.2.1:
     graceful-fs "^4.1.2"
     write "^0.2.1"
 
-flat@^2.0.1:
-  version "2.0.1"
-  resolved "https://registry.yarnpkg.com/flat/-/flat-2.0.1.tgz#70e29188a74be0c3c89409eed1fa9577907ae32f"
+flat@^4.0.0:
+  version "4.0.0"
+  resolved "https://registry.yarnpkg.com/flat/-/flat-4.0.0.tgz#3abc7f3b588e64ce77dc42fd59aa35806622fea8"
   dependencies:
-    is-buffer "~1.1.2"
+    is-buffer "~1.1.5"
 
 flatten@^1.0.2:
   version "1.0.2"
@@ -3861,10 +3861,14 @@ is-binary-path@^1.0.0:
   dependencies:
     binary-extensions "^1.0.0"
 
-is-buffer@^1.1.5, is-buffer@~1.1.2:
+is-buffer@^1.1.5:
   version "1.1.5"
   resolved "https://registry.yarnpkg.com/is-buffer/-/is-buffer-1.1.5.tgz#1f3b26ef613b214b88cbca23cc6c01d87961eecc"
 
+is-buffer@~1.1.5:
+  version "1.1.6"
+  resolved "https://registry.yarnpkg.com/is-buffer/-/is-buffer-1.1.6.tgz#efaa2ea9daa0d7ab2ea13a97b2b8ad51fefbe8be"
+
 is-builtin-module@^1.0.0:
   version "1.0.0"
   resolved "https://registry.yarnpkg.com/is-builtin-module/-/is-builtin-module-1.0.0.tgz#540572d34f7ac3119f8f76c30cbc1b1e037affbe"