Translate file input

• On mobile, the import form takes more than the full screen's width, at least on Firefox responsive mode. @nicofrand can you have a look, please? I tried a couple things but didn't succeed :/
• The file input could be split back into a customized component for file inputs, so that it's properly translated. Would be done by hiding the actual file input and using a button (with translation) + label (file name) instead.
• move the background-color and color for the file input CSS code to theme files (that was @nicofrand's idea!).