Under questions for the opportunities, when someone chooses a file upload for their question, provide the ability to select a particular file type. For example, if an Opportunity Administrator only wants PDF files submitted allow the person to select PDF only. Or if they only want a Word Doc, allow that to be one of the options. If they don't have a preference, allow all the file types to be allowed. Also, provide help text so that anyone who is uploading a file will know not to include punctuation, special characters, spaces in the file name so that they know if they have any of this in their documents title that the system will not accept the document. Also include to users which file types are not allowed. Specify the acceptable file type when setting up the question, along with letting file uploaders know what is an acceptable title for their document and which document types are acceptable.