fix: add .dat file extension automatically when exporting watchonly

[polespinasa]

fixes #956

Unlike backup wallet, export watch-only wallet was not automatically adding the file extension to the exported file, making restoring difficult if the user doesn't manually add the file extension after exporting.

Allows also to restore a wallet from a non specified .dat file extension. This is achieved by removing the filter in the select file screen, matching the RPC behavior.

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[hebasto]

Unlike backup wallet, export watch-only wallet was not automatically adding the file extension to the exported file, making restoring difficult if the user doesn't manually add the file extension after exporting.

As I noted here:

The GUI behaviour is consistent with the RPC.

This change does not prevent using exportwatchonlywallet no_dat_extension RPC and subsequently opening the file in the GUI.

[polespinasa]

This change does not prevent using exportwatchonlywallet no_dat_extension RPC and subsequently opening the file in the GUI.

Right, probably more correct to say partially-fixes, this PRs tries to mimic what backup wallet does. Which does add the file extension by default when using the gui.

I think the normal non-technical user (will never use the RPC) workflow is just using the GUI options, and that should not imply adding some unknown data file extensions manually. A user might not know what a .dat is. Adding it by default seems the correct way to me.

[polespinasa]

This change does not prevent using exportwatchonlywallet no_dat_extension RPC and subsequently opening the file in the GUI.

Now it does :)

Second commit allows to load a non .dat file. Force file type filtering might cause the issue reported in #956, thus breaking UX.

Still I think the first commit is good as adding the file extension is normally good expected behavior and is what the GUI already does with backup wallet.

[achow101]

ACK 10ea03e