Add a warning when the attachment extension is modified

Currently, no control are applied the new attachment name when moving an attachment.

This means that an user making a mistake when editing the attachment name won't be warned.

It can be interesting to add a verification step of the file extension before proceeding to the move.

Note: Another way to present this is to only select the part before the extension when clicking on the name field (and to select all on an additional double click)