Allow me to select a file and upload it, in a discoverable way: from the menu.

### Is your feature request related to a problem? Please describe.

The case is that for instance I download something in telegram or export a backup from some app on my phone . I then have to struggle to get the file off of my phone to somewhere I can use it on my desktop.

### Describe the solution you'd like

I want to be able to open the app, see my local filesystem and select a file to upload to my attached nextcloud.

### Describe alternatives you've considered

The only option I found is to monitor an entire directory (like your DCIM folder) which seems to be the only way for me to upload stuff and it doesn't fit the one-off way of doing things.