I can tell you with ABSOLUTE certainty that a random filename is being used. I was refactoring the attachments code ten days ago. The entire architecture of attachment is very solid in my working memory right now. You can verify it by looking in the media/com_ats/attachments folder. See all those funny looking filenames in there? Yup. These are your attachments, not garbage. The mapping between real files and these "mangled" filenames is in your database, in the #__ats_attachments table.
Also, the name of the file you are trying to attach is absolutely valid on Linux. Linux is very permissive with regards to filenames. You can use pretty much anything except a null byte! It's Windows that has a bazillion restrictions about filenames.
You misdiagnosed your problem. Your problem is very different and has actually nothing to do with Akeeba Ticket System itself. That's why I asked you to explain your use case.
All attachments are uploaded using Joomla's core API for file uploads. By default, Joomla applies various security checks against the uploaded files. One of the checks is "does the filename of the uploaded file, as reported by the browser, contain more than one dots?". If it does, it's automatically rejected. When Joomla rejects the upload for security reasons –for ANY of its security checks, not just the multiple dots– ATS does not attach the file.
Your filename is Groupe Pac air-eau ext..pdf See the two dots before pdf? That's what triggered Joomla's upload security and caused your file to be rejected.
When you renamed the file to shema.pdf you fixed the double dot issue and the upload worked.
Your problem, therefore, has nothing to do with ATS. It's just Joomla's file upload security being triggered :)
Nicholas K. Dionysopoulos
Lead Developer and Director