diff --git a/matrix-sdk-android/src/main/java/im/vector/matrix/android/internal/session/content/UploadContentWorker.kt b/matrix-sdk-android/src/main/java/im/vector/matrix/android/internal/session/content/UploadContentWorker.kt
index 2597ef4e31..90d15cc3b2 100644
--- a/matrix-sdk-android/src/main/java/im/vector/matrix/android/internal/session/content/UploadContentWorker.kt
+++ b/matrix-sdk-android/src/main/java/im/vector/matrix/android/internal/session/content/UploadContentWorker.kt
@@ -86,7 +86,7 @@ internal class UploadContentWorker(context: Context, params: WorkerParameters) :
                 uploadedThumbnailEncryptedFileInfo = encryptionResult.encryptedFileInfo
 
                 fileUploader
-                        .uploadByteArray(encryptionResult.encryptedByteArray, "thumb_${attachment.name}", thumbnailData.mimeType)
+                        .uploadByteArray(encryptionResult.encryptedByteArray, "thumb_${attachment.name}", "application/octet-stream")
             } else {
                 fileUploader
                         .uploadByteArray(thumbnailData.bytes, "thumb_${attachment.name}", thumbnailData.mimeType)