所以我找到了一个变通办法：而不是尝试将文件直接发送到firebase，我可以

• 首先将文件作为字节数组读入RAM
• 然后将字节数组发送到firebase

下面是一些代码：

FirebaseStorage storage;
        StorageReference storageRef;

        storage = FirebaseStorage.DefaultInstance;
        storageRef = storage.RootReference;

        byte[] fileData = File.ReadAllBytes(localFilePath);
        StorageReference storageDataRef = storageRef.Child(firebaseFilename);
        storageDataRef.PutBytesAsync(fileData)
            .ContinueWith((Task<StorageMetadata> task) => {
                if (task.IsFaulted || task.IsCanceled) {
                    Debug.Log(task.Exception.ToString());
                    // Uh-oh, an error occurred!
                }
                else {
                    // Metadata contains file metadata such as size, content-type, and md5hash.
                    StorageMetadata metadata = task.Result;
                    string md5Hash = metadata.Md5Hash;
                    Debug.Log("Finished uploading...");
                    Debug.Log("md5 hash = " + md5Hash);
                }
            });

现在我没有得到任何“FileContentProvider”错误了。不知道为什么我必须这样做虽然...