Odoo 10 strange behaviour : if I want to attach a PDF or a text file to an expense report, no prob, the number of documents up right is right and I retrieve my uploaded docs.

If I want to upload another file format, it seems that there's no problem, everyhthing goes fine until...the number of documents is zero !

If I go and see all attachments list (with devel mode activated), all my files are there... So it seems that only specific file formats are allowed to be attached to an expense report but I didn't find any clue about this, no doc online tells about that, and no, the open file dialog which appears at the time of the upload doesn't say anything about that either.

Is it a normal behaviour ? Should I be allowed to upload any file format ? Is there somewhere a list of allowed file formats or extensions ? Thanks