No, it is not possible to view attachments that are added to a record of a related table directly from a feature service after the service has been added to ArcMap. However, attachments added to related records can be viewed after exporting the feature service to a file geodatabase and viewing the exported data in ArcMap.
Although ArcMap does not allow viewing attachments from related table records of feature services, it is possible to view these types of attachments using ArcGIS Pro or ArcGIS Online. In ArcGIS Online, the attachments can be viewed using the Data tab of the Item Details of a feature service.
In ArcGIS Pro, related table attachments from a feature service can be viewed through the Attributes pane after selecting a related record. Additionally in ArcGIS Pro, a related table from the feature service can be opened and after right-clicking on a selected record, use Manage Attachments to view the attachment.
It is possible to view attachments that have been added to a feature, rather than a related record, of a feature service in ArcMap. When the feature layer is associated with attachments, the attachment of a particular feature can be viewed in ArcMap using the HTML pop-up tool or by using the Identify tool. Another option to view feature layer attachments is to open the attribute table, right-click a particular record, and select Open Attachment Manager. When working with related tables from a feature service, these options are not available for viewing attachments in ArcMap.
To view attachments in related tables from a feature service in ArcMap, a copy of the source file geodatabase should be downloaded or accessed. The source file geodatabase for a hosted feature service can be downloaded from the Item Details page using the option to Export Data. Alternatively, the option to Create Replica can be used from the REST page of the hosted feature service.