I have tried to load a PostGIS layer containing a binary data field (BLOB, bytea) and then use the new form widget for binary fields (https://qgis.org/en/site/forusers/visualchangelog36/index.html#feature-new-form-widget-for-binary-blob-fields). The problem is, that the binary field will not be in the layers attribute table, so the widget can not be used/activated. In the QGIS database manager the field is visible in the table preview. Does anybody knows what is going wrong or what I am missing?
QGIS (as of version 3.8.0) has no support for binary/bytea fields from Postgres sources. As of version 3.8, they are only supported for disk-based vector layers and in-memory scratch layers.