This only happens in non-English installations (fre-FR in this case), and in FF. Other browsers aren't affected.

Steps to reproduce:

• Prepare a test installation with fre-FR as the main language, and fully updated;
• Login to the admin interface in Firefox;
• Select a content and click on "Upload multiple files" - "Envoyer plusieurs fichiers" in this case, since the admin interface is in fre-FR;
• See that the "Select Files" button isn't rendered correctly (see attached "ez_support_issue_6095_screenshot_1_ff.png" screenshot);
• Problem does not occurr on other browsers (see attached "ez_support_issue_6095_screenshot_2_chrome.png" screenshot);