File existence check.

### Is your feature request related to a problem? Please describe.
I have configured to move all pictures to server. Some of the pictures haven't deleted, I actually don't know why. Maybe upload error or file conflict.

### Describe the solution you'd like
To make sure the pictures are in the server, please add a feature while sharing files with the nextcloud to have an option to check existence. The list of the files that does not exist can be uploaded or can be selected to copy it elsewhere on the device.

### Describe alternatives you've considered
Otherwise it is hours to check each picture for existence on server, as each picture sorted by year/month/day folders.

### Additional context
The feature needs to work while sharing list of files with the nextcloud.

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