Attachments and file sharing

Attachments are any files that you send or receive enclosed in an e-mail.

Avoid sending unnecessary attachments as they take up space in both your and the recipient’s mailbox.  In addition, attachments may be difficult to find in mailboxes and cannot be edited from the mailbox.

Use well-known file formats for your attachments so that the recipient will be able to open them. Read more about file types in chapter 1.2.

If possible, instead of using an attachment, type your message in the message text field or add a link to a file stored elsewhere on the network. Various cloud services (such as OneDrive included in the Office 365 package) are usually better suited for the purpose. Such services offer better functionalities for rights management as well as for multi-party editing and commenting. This usually also makes teamwork easier and faster. Read more about this in chapter two.

Recommended file size

Many e-mail programs block large attachments altogether in order to prevent them from using up all the space on servers or in mailboxes. The size limit varies between systems (it could be 5 MB, 10 MB or more).

If a message contains an oversize attachment, the message may “bounce” back to the sender without ever being transmitted to the recipient. The reason for this may be a restriction of the receiving mail server, for example. Another reason could be that the mail server does not accept the file type of the attachment.

  • In most cases, the best option is to share the file using a cloud service such as the University of Helsinki OneDrive for Business. It also lets you easily share folders and invite others to work on projects and provides you with better tools for controlling your file sharing settings.
  • Files of hundreds of megabytes or even gigabytes should be shared through a file sharing service, such as Funet Filesender.

Handling attachments

Usually you can receive and send attachments with all e-mail programs.  Bear in mind that attachments may contain viruses. Do not open an attachment unless you are sure of its contents!


To add an attachment in Outlook, create a new e-mail and click the Attach File button in the message window. The program displays a file selection dialog box. Select the file you want and click Insert. The program adds the file to your message. It is polite to write a concise accompanying note about the attachment (see the image below). You can then send the message as usual.


An image of a paper clip, or some other similar symbol tells you that the message contains an attachment.


Receiving and opening

You can open attachments as follows:

  • save the attachment to a folder on your computer and open it manually in the folder
  • click the attachment icon in the e-mail message.

The preferred way is to save the attachment to your computer. If you open the attachment directly from the message, it may open in a wrong application, or in read-only mode – or it may not open at all.

If you open an attachment directly from the network or the e-mail program, it is saved to a default temporary folder specified in the e-mail program. Remember to save the attachment to your own work folder if you want to edit it!

Office files opened from the Internet or from e-mail attachments open in a read-only mode by default. If you want to edit the attachment, click the Enable Editing button shown in the image below.

To save a file in Outlook, right-click the attachment icon and choose Save as in the folder that opens.

Handling suspicious attachments

If you suspect that the attachment contains a virus or malware, it often does. If you are not sure that the attachment is safe, delete the message and attachment. Bear in mind the following:

  • That you recognize the sender is no guarantee of the safety of the attachment! Many viruses are able to forge the name of the sender and can send you messages and attachments which look genuine.
  • Attachments with viruses are usually disguised as tempting and appealing: the message typically tells you about a tragedy, love, winning in the lottery or some other emotionally appealing subject to get you to open the attachment by mistake.
  • With attachments, it is wise to be suspicious. Consider whether the sender of the message gives credible reasons for sending the attachment. The language of the message, the sender or the attachment icon are easy to forge. Even a message in Finnish from a friend may contain viruses!

Many e-mail systems (also at the University of Helsinki) prevent sending and receiving certain file types. File types not allowed often include many executable files with file extensions such as exe, bat, com, scr, vbf or pif. Nevertheless, always check the file type of the attachment! Pay special attention to attachments with two file extensions. The first extension is usually there just to fool you into thinking that the file is genuine. A file with two file extensions may look like this, for example: Photo of my girlfriend.jpeg.exe