trigger script (to convert to epub) after upload and before add to database

[RuBiCK]

Is your feature request related to a problem? Please describe.
Amazon will only accept epub files. It doesn't make sense for kindle users to add mobi files to the library.

Describe the solution you'd like
It would be great the possibility to upload and automatically convert if the ebook file is other than epub

Describe alternatives you've considered
This could be done in two different ways.

1. with an option in admin config, when activated and a new file is uploaded, it will use the binary to convert, from any book to epub after upload and before adding to database, saving the output file to database
2. the easy way (less development I think) is to let the user execute a script after upload and before add to database. It's like option 1 but the logic it's implemented by the user. This could be named trigger script before upload or trigger script instead upload

[OzzieIsaacs]

This would be something for the planned and yet not implemented plugin system: #1566