Many-to-many-relations: support multi-file-upload

[BlackbitDevs]

For (advanced) many-to-many relations with assets being allowed you can directly upload assets from the relation view:

Currently this only supports single-file uploads. The same applies under assets context menu for "Upload (Compatibility mode)". With this PR all those places now support multi-file upload.

[robertSt7]

@BlackbitDevs Could you please replace var with let and const. Thank you

[sonarqubecloud]

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[kingjia90]

One minor graphical thing is that the popup that appears when clicking on the upload button, it doesn't hint that you can select one or more files

[BlackbitDevs]

One minor graphical thing is that the popup that appears when clicking on the upload button, it doesn't hint that you can select one or more files

Actually people will assume this as they know it from other applications, won't they?
Of course we could change the placeholder to "Select files" - this should include one or multiple. WDYT?

[kingjia90]

Yes, that's right, the text change would be fine to me

[BlackbitDevs]

Changed to "Select files" - the non-english translations are still maintained outside of github, right?

[kingjia90]

Yes, but i would use a new translation key instead.

[sonarqubecloud]

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