Save confirmation popup does not save on custom binary editor

[safisa]

Bug Description:

Hi,
We are using vscode extension with custom editors, and some are based on a binary data model (CustomEditorProvider). In the latest releases there is a regression in the save confirmation popup when pressing save on a dirty/modified editor.
I have found that the PR caused this issue is back to Theia 1.33 - PR #11261
Once going back one commit before this change, the save works!

Note that using the custom text model (CustomTextEditorProvider, eg. example.cscratch) is working.

Steps to Reproduce:

1. use the famous cat-customs-0.0.2.vsix extension sample
2. open the example.pawDraw (binary format)
3. do some changes
4. see that the tab/file is in dirt mode
5. close the tab, and a save confirmation will popup
6. press save
7. reopen the file
8. file is still the same (save didn't work)

Additional Information

• Operating System: mac/win
• Theia Version: 1.33-1.40