Avoid Large Attachments and Certain Attachment Types
In general, .jpg, .gif, .png and .pdf attachments are safe to send, provided you include some content in the email as well. However, executable attachments such as .exe, .zip, .swf, etc. should be avoided entirely. Generally, you should not send attachments to people on your list that are not expecting them.
If you need to email a large attachment or an attachment type that usually can be flagged as spam or trigger virus scanners, we recommend a service such as DropBox.com. If the attachment contains sensitive data, you may consider using your company’s secure FTP server.
From – https://sendgrid.com/blog/10-tips-to-keep-email-out-of-the-spam-folder/