This can often be the case if you have saved your document in the wrong file format. You should find that all software is available on vdi.soton.ac.uk where you can then open and resave your document into the file format needed.
Alternatively this can be a common issue for Word (.doc/.docx) or Excel (.xls/.xlsx) based documents. Because of this we highly recommend that you do not use any other software to create a document with such as OpenOffice, Google Docs and Apple Pages.
Always use Microsoft Office products to create and save your documents in. As a student you get 5 free copies of MS Office suite programs from your Office 365 account and you can always use vdi.soton.ac.uk if needs be.
If you are still unable to upload the document check with your Faculty Office to make sure you are trying to upload the correct file format.