Office 365 users can't open or view attachments in Outlook Web App

Applies to: Exchange Online

PROBLEM


When Office 365 users try to open or view attachments in email messages in Outlook Web App, they experience the following symptoms:
  • In Office 365, the attachment isn't displayed in the message. Instead, a generic placeholder, such as “1 Attachment” is displayed, and this can't be opened.
  • When users try to view attachments in Office 365, they receive the following error message: 
    Access to attachments has been blocked. Blocked attachments: <FileName>.

CAUSE


This issue occurs if the attachment is blocked by Outlook Web App. By default, Outlook Web App blocks attachments that have the following file name extensions:
.vsmacros, .msh2xml, .msh1xml, .ps2xml, .ps1xml, .mshxml, .gadget, .mhtml, .psc2, .psc1, .msh2,.msh1, .aspx, .xml, .wsh, .wsf, .wsc, .vsw, .vst, .vss, .vbs, .vbe, .url, .tmp, .shs, .shb, .sct,.scr, .scf, .reg, .pst, .ps2, .ps1, .prg, .prf, .plg, .pif, .pcd, .ops, .mst, .msp, .msi, .msh,.msc, .mht, .mdz, .mdw, .mdt, .mde, .mdb, .mda, .maw, .mav, .mau, .mat, .mas, .mar, .maq, .mam,.mag, .maf, .mad, .lnk, .ksh, .jse, .its, .isp, .ins, .inf, .htc, .hta, .hlp, .fxp, .exe, .der,.csh, .crt, .cpl, .com, .cmd, .chm, .cer, .bat, .bas, .asx, .asp, .app, .adp, .ade, .ws, .vb, .js

SOLUTION


Change the Outlook Web App mailbox policy to include and exclude the file types that you want. For more information about how to do this, see the “Add or remove file types from file access lists” section of the following Microsoft website: The following is an example of the Windows PowerShell commands to remove the .xml file type from the BlockedFileTypes and BlockedMimeTypes lists and add it to the AllowedFileTypes and AllowedMimeTypes lists:
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedFileTypes @{Remove = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -AllowedFileTypes @{Add = ".xml"}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedMimeTypes @{Remove = "text/xml", "application/xml”}
Get-OwaMailboxPolicy | Set-OwaMailboxPolicy –AllowedMimeTypes @{Add = "text/xml", "application/xml”}
Things to consider:
  • It may take several minutes before changes to the Outlook Web App policy take effect.
  • Be aware that by changing the Outlook Web App mailbox policy to include file types that are blocked by default, you may make your system more vulnerable to security threats.

WORKAROUND


Direct users to compress the files that they intend to send (for example, as a .zip file) and then send the compressed files as attachments.

MORE INFORMATION


Still need help? Go to Microsoft Community.