javax.mail.MessagingException: Missing start boundary

Hello, We are facing some problems with the uploading zip functionnality (with Firefox 45.3 at least) : 1) Steps to reproduce: - Open Firefox 45.3.0 ESR with Windows 7 - Upload a zip archive file with this kind of input: <input type="file" accept=".zip" value="" id="xx" name="xx"> 2) Actual results: 2 strange things : - On some workstations (not all), for the same versions of Firefox (45.3), the Content-Type associated with the file is "multipart/x-zip". Why specially this content-type ? Others more commons Content-Type ("application/x-zip" or "application/octet-stream") are usually sending I guess. - The Content-type "multipart/x-zip" is sending without boundary part, causing the following error on our servers : javax.mail.MessagingException: Missing start boundary at javax.mail.internet.MimeMultipart.parsebm(MimeMultipart.java:882) ~[javax.mail.jar:1.4.5] at javax.mail.internet.MimeMultipart.parse(MimeMultipart.java:503) ~[javax.mail.jar:1.4.5] at javax.mail.internet.MimeBodyPart.getContent(MimeBodyPart.java:652) ~[javax.mail.jar:1.4.5] 3) Expected results: - Either a more common Content-Type for archive zip file - Either a boundary part with "multipart/x-zip" Content-Type Someone have an explanation for this behaviour ? Thanks for your support !