For disabling default file format dialog, you may:
Seting default file format to MS XML in registry:
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\General]
"FirstRunTime"=dword:019975fa
"FileFormatBallotBoxAppIDBootedOnce"=dword:00000000
"FileFormatBallotBoxTelemetryEventSent"=dword:00000001
"ShownFileFmtPrompt"=dword:00000001
"FileFormatBallotBoxShowAttempts"=dword:00000001
For Open XML:
[HKEY_CURRENT_USER\SOFTWARE\Microsoft\Office\16.0\Common\General]
"FirstRunTime"=dword:019975fb
"FileFormatBallotBoxAppIDBootedOnce"=dword:00000000
"FileFormatBallotBoxTelemetryEventSent"=dword:00000001
"ShownFileFmtPrompt"=dword:00000001
"FileFormatBallotBoxShowAttempts"=dword:00000001
Or set this in XML file which is used for install office 365 by command
setup" /configure office365-install.xml
<AppSettings>
<User Key="SOFTWARE\Microsoft\Office\16.0\Common\General" Name="FirstRunTime" Value="019975fa" Type="REG_DWORD" App="office16" />
<User Key="SOFTWARE\Microsoft\Office\16.0\Common\General" Name="FileFormatBallotBoxAppIDBootedOnce" Value="00000000" Type="REG_DWORD" App="office16" />
<User Key="SOFTWARE\Microsoft\Office\16.0\Common\General" Name="FileFormatBallotBoxTelemetryEventSent" Value="00000001" Type="REG_DWORD" App="office16" />
<User Key="SOFTWARE\Microsoft\Office\16.0\Common\General" Name="ShownFileFmtPrompt" Value="00000001" Type="REG_DWORD" App="office16" />
<User Key="SOFTWARE\Microsoft\Office\16.0\Common\General" Name="FileFormatBallotBoxShowAttempts" Value="00000001" Type="REG_DWORD" App="office16" />
</AppSettings>
Instalator put this setings from XML in registry:
Počítač\HKEY_LOCAL_MACHINE\SOFTWARE\Microsoft\Office\16.0\User Settings{SOMENUMBER}\Create\software\microsoft\office\16.0
(Number in this key is not the same for all instalations.)
When user run first time office aplication like Winword, office copy this registry keys in your HKCU.
Default file format is now set, and dialog is not shown.