Can GrapesJS allow uploading arbitrary file through the built in asset manager?

Good question, I think the issue is the OpenAssets command itself that should be renamed in OpenImageAssets (indeed it renders only images types). So, what you can try to do is to create another command (eg. a more generic one or just for documents) and see if the customization works out for you. If you will see any point of improvement in the process I'll be glad to discuss it.

@artf I created the referenced pull request. I admit my motivation was my specific use case, but hopefully it was coded generically enough to be acceptable in the core editor codebase.

... oh and just to clarify - another chunk of code written alongside this pull request resides in a custom plugin of my project. It's a plugin that essentially does the following:

1. Customises the Link DOM component such that upon creating a link via the RTE, the asset manager opens up at which point an arbitrary asset can be uploaded and linked to the newly created link component.
2. Customises the Image asset type of the asset manager such that upon selection of an image in the asset manager, either the src or href attribute of the target component get updated.
3. Adds an Other asset type to the asset manager. The Other asset type is used alongside the Image asset type if the asset manager is used for the Link DOM component.