diff --git a/__tests__/utils/upload.spec.ts b/__tests__/utils/upload.spec.ts
index 02299147..5ec362b1 100644
--- a/__tests__/utils/upload.spec.ts
+++ b/__tests__/utils/upload.spec.ts
@@ -117,6 +117,9 @@ describe('Upload data', () => {
 			data: blob,
 			signal,
 			onUploadProgress,
+			headers: {
+				'Content-Type': 'application/octet-stream',
+			},
 		})
 	})
 	test('Upload async data stream', async () => {
@@ -139,6 +142,9 @@ describe('Upload data', () => {
 			data: blob,
 			signal,
 			onUploadProgress,
+			headers: {
+				'Content-Type': 'application/octet-stream',
+			},
 		})
 	})
 
@@ -161,6 +167,7 @@ describe('Upload data', () => {
 			onUploadProgress,
 			headers: {
 				Destination: url,
+				'Content-Type': 'application/octet-stream',
 			},
 		})
 	})
diff --git a/lib/uploader.ts b/lib/uploader.ts
index 9bdda4b7..f6bcc76a 100644
--- a/lib/uploader.ts
+++ b/lib/uploader.ts
@@ -215,6 +215,7 @@ export class Uploader {
 							{
 								'X-OC-Mtime': file.lastModified / 1000,
 								'OC-Total-Length': file.size,
+								'Content-Type': 'application/octet-stream',
 							},
 						)
 							// Update upload progress on chunk completion
@@ -284,6 +285,7 @@ export class Uploader {
 							undefined,
 							{
 								'X-OC-Mtime': file.lastModified / 1000,
+								'Content-Type': file.type,
 							},
 						)
 
diff --git a/lib/utils/upload.ts b/lib/utils/upload.ts
index 5a9c133e..259f111f 100644
--- a/lib/utils/upload.ts
+++ b/lib/utils/upload.ts
@@ -11,21 +11,41 @@ type UploadData = Blob | (() => Promise<Blob>)
 
 /**
  * Upload some data to a given path
+ * @param url the url to upload to
+ * @param uploadData the data to upload
+ * @param signal the abort signal
+ * @param onUploadProgress the progress callback
+ * @param destinationFile the final destination file (often used for chunked uploads)
+ * @param headers additional headers
  */
-export const uploadData = async function(url: string, uploadData: UploadData, signal: AbortSignal, onUploadProgress = () => {}, destinationFile: string | undefined = undefined, headers: any = undefined): Promise<AxiosResponse> {
+export const uploadData = async function(
+	url: string,
+	uploadData: UploadData,
+	signal: AbortSignal,
+	onUploadProgress = () => {},
+	destinationFile: string | undefined = undefined,
+	headers: Record<string, string|number> = {},
+): Promise<AxiosResponse> {
 	let data: Blob
 
+	// If the upload data is a blob, we can directly use it
+	// Otherwise, we need to wait for the promise to resolve
 	if (uploadData instanceof Blob) {
 		data = uploadData
 	} else {
 		data = await uploadData()
 	}
 
+	// Helps the server to know what to do with the file afterwards (e.g. chunked upload)
 	if (destinationFile) {
-		headers ??= {}
 		headers.Destination = destinationFile
 	}
 
+	// If no content type is set, we default to octet-stream
+	if (!headers['Content-Type']) {
+		headers['Content-Type'] = 'application/octet-stream'
+	}
+
 	return await axios.request({
 		method: 'PUT',
 		url,
diff --git a/package.json b/package.json
index 87447a2b..ec6adbf8 100644
--- a/package.json
+++ b/package.json
@@ -35,7 +35,7 @@
     "cypress": "cypress run --component",
     "cypress:gui": "cypress open --component",
     "test": "vitest run",
-    "test:watch": "vitest run --watchAll --verbose true",
+    "test:watch": "vitest run --watch",
     "test:coverage": "vitest run --coverage",
     "l10n:extract": "node build/extract-l10n.js"
   },