It is usually a simple matter to remedy, the dossier file is still safe and secure in the database.
The quick method to fix it: Upload it again making sure you save it with the correct file type.
Here is the technical reason for the problem and why a repeat of the upload procedure works:
As you are aware your computer (PC) can identify various different types of files. For example, you most likely use MS-Word for your typed documents. So when you get a Word document you simply click on it and it opens automatically in Word. Other examples are MS-Excel, pictures such as bitmaps and scanned documents ("jpeg" files). Your computer usually has no problem with most of these file types. When it cannot identify the file type it can even give you a list of file types to choose from.
When a dossier is stored on the database, the PC "tells" DevMan what file type it is uploading and this information is kept with the file. When you later open the file, the PC (Web browser) uses the stored information to identify the file type and thus which program to use to open it.
If the dossier was stored (uploaded) on a PC that didn't know the file type, or provided the wrong file type (because it was registered incorrectly) then it is likely that other PCs will not be able to open the attachment. However when you upload it again, it works because your PC (or the fixed PC) is providing the correct type.