Yes, this is true. I saw the example in the webdav-documents-browse screen, but this is not the intended behavior.
What I want to achieve is to have the upload action in the actions menu for a table. The approach you suggested seems to be confusing for my client and the need is to be able to upload a new version of a document using the context action menu.
Is it possible to open a file upload dialog through an action and somehow use the FileStorageUploadField functionality for storing the file?
Unfortunately, it is impossible to upload a file without FileStorageUploadField because it is a wrapper over <input type="file"/>. As a possible solution, I suggest creating a screen with FileStorageUploadField and show it in a dialog after invoking an action. When screen is closed, obtain information about a file (see doc).