[Submissions] Check extension of uploaded documents
According to our guidelines, authors should send their manuscripts (both for the first submission and resubmissions) as a pdf file when using SciPost's preprint server. However, our systems don't enforce this and occasionally we receive tex or doc files, which derails the editorial process. The system should check the extension of a file in order for a submission to be successful, and fail the submission of other formats.
We could consider implementing a similar filter for attachments to referee reports and comments, although for those we may want to allow png and jpg formats.