Fix file transfert and notification behaviour

- Add External Image Provider to avoid having error message when we try to show a file that is not an image (used to show an attachment where it can be a regular file).
- When creating an image, try to read the headers of a file when the extension is not consistent with contents.
- Build if not exists, a thumbnail image if the file has been displayed when filling a message
- When trying to open the file, test if it was downloaded by reading the flag
- Add a status for File Transfert In Progress
- Use non-deprecated function to get file path from ChatMessage
- Use FileSize in addtion of content Size to get the full size of a message
- Show a progressbar when downloading a file
- Show the image in notification when downloaded or the file name if it is not an image