Forms store upload files in "Local Asset Folder" of the [File Upload] element. So you can find the element in your form, then list all the content below "Uploaded Files" folder, you will get all the upload files. Below is how you can find the folder:

var assetFolder = _contentAssetHelper.Service.GetOrCreateAssetFolder(elementContentLink);
var folder = _contentRepository.Service.GetChildren<ContentFolder>(assetFolder.ContentLink)
                .FirstOrDefault(f => f.Name.Equals(Constants.FileUploadFolderName));

elementContentLink in this case is your File Upload element's content link.

Thanks for your reply. I marked it as the answer for my question. I had to change 

_contentRepository.Service.GetChildren<ContentFolder>

to

_contentRepository.Service.GetChildren<MediaData>

In order to get my solution to work as expected.