Are you using manifest v3? We did introduce the simpler DOM File return value for manifest v3. https://searchfox.org/comm-esr115/rev/8c0636366d68bd854091e40b984e4b72c2dc1779/mail/components/extensions/parent/ext-messages.js#757-767 Since we do not yet officially support manifest v3, the patch was backported as a whole to allow using DOM File option in manifest v2 already as well.
Bug 1859854 Comment 1 Edit History
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Are you using manifest v3? We did introduce the simpler DOM File as the default return value for manifest v3. https://searchfox.org/comm-esr115/rev/8c0636366d68bd854091e40b984e4b72c2dc1779/mail/components/extensions/parent/ext-messages.js#757-767 Since we do not yet officially support manifest v3, the patch was backported as a whole to allow using DOM File (optionally) in manifest v2 already as well.
Are you using manifest v3? We did introduce the simpler DOM File as the default return value for manifest v3. https://searchfox.org/comm-esr115/rev/8c0636366d68bd854091e40b984e4b72c2dc1779/mail/components/extensions/parent/ext-messages.js#757-767 Since we do not yet officially support manifest v3, the patch was backported as a whole to allow using DOM File (optionally) in manifest v2 already as well. Edit: The documentation still needs to be updated, expect that to happen within the next few days.