Allow more than one file in "Upload Format" operation

[jcarloscamacho]

Is your feature request related to a problem? Please describe. Currently the "Upload Format" button opens a file dialog that only allows to select one file at a time.

Describe the solution you'd like It would be desirable to upload several versions of a book (epub, mobi, etc) at the same time, because it's tedious to do it one by one.

Describe alternatives you've considered If not possible to change the dialog option to multiple select, allow the ebook-form to add several formats each time you press the "Upload Format" button and select a file. Have some kind of queue so when you press "Save" button all the formats selected will be uploaded and saved.

Additional context No additional context.