Hi Steven,
Since it does not look like Mendix Marketplace/App Store has any widget for this, it may be necessary for you / someone else to create such widget that can allow storing images on paste from your clipboard.
Although this requires a fair amount of knowledge in programming in JavaScript/similar languages that Mendix supports for making widgets.
Here are some links to get started with writing code that can get the image from your clipboard when pasting:
https://jsfiddle.net/ourcodeworld/hzvfq82b/
These are for images, but files/documents should also be possible to obtain :-)
To store file data to a FileDocument (or a sub-entity of it): https://apidocs.rnd.mendix.com/8/client/mx.data.html#.saveDocument
Additionally, you can find documentation to get started with writing Mendix widgets here:
https://docs.mendix.com/howto8/extensibility/widget-development
https://github.com/mendix/AppStoreWidgetBoilerplate
There’s also pluggable web/native widgets now, but it is not something I have worked with.
I hope this helps you,
Andreas
assuming there are still no widgets that allow you to paste an image?