If you do not see it on the feature class in the geodatabase you can also enable this on the hosted feature service in ArcGIS Online.
From the doc:
Manage hosted web layers—ArcGIS Online Help | ArcGIS
Allow attachments
You can allow editors to attach images and other files to individual features in the feature layer. This is useful, as it allows you to share documentation or photos that are relevant to specific features. For example, a code compliance officer might attach a photo of the code violation for a specific address point, or a building inspector might include a PDF of a permit for a building feature.
The file you attach to a feature can be up to 10 MB in size. For a list of supported file types, see Edit features. Be aware that the size of the attachments adds to the size of the feature layer.
Follow these steps to allow people to attach files to features in your feature layer:
- You must first enable editing on the hosted feature layer.
- While still signed in to your ArcGIS Online account, open the feature layer's details page by clicking the layer in My Content.
- Click the small arrow icon next to the layer for which you want to allow attachments and choose Enable attachments.
Now, when people edit the feature layer, they can attach files to individual features.
If you decide you no longer want files to be attached to your feature layer, you can click the small arrow icon next to the layer in the feature layer's details page and choose Disable attachments. This will drop all existing attachments and prevent people from adding any more attachments.