Microsoft 365 users can't open or view attachments in Outlook on the web

Problem:

When Microsoft 365 users try to open or view attachments in email messages in Outlook on the web, they experience the following symptoms:

in Microsoft 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 Microsoft 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 on the web. By default, Outlook on the web blocks attachments that have the following file name extensions:

.ade, .adp, .apk, .app, .appcontent-ms, .application, .appref-ms, .appx, .asp, .aspx, .asx, .bas, .bat, .bgi, .cab, .cdxml, .cer, .chm, .cmd, .cnt, .com, .cpl, .crt, .csh, .der, .diagcab, .exe, .fxp, .gadget, .grp, .hlp, .hpj, .hta, .htc, .img, .inf, .ins, .iso, .isp, .its, .jar, .jnlp, .js, .jse, .ksh, .lnk, .mad, .maf, .mag, .mam, .maq, .mar, .mas, .mat, .mau, .mav, .maw, .mcf, .mda, .mdb, .mde, .mdt, .mdw, .mdz, .mht, .mhtml, .msc, .msh, .msh1, .msh2, .mshxml, .msh1xml, .msh2xml, .msi, .msp, .mst, .msu, .ops, .osd, .pcd, .pif, .pl, .plg, .prf, .prg, .printerexport, .ps1, .ps1xml, .ps2, .ps2xml, .psc1, .psc2, .psd1, .psdm1, .pssc, .pst, .py, .pyc, .pyo, .pyw, .pyz, .pyzw, .reg, .scf, .scr, .sct, .settingcontent-ms, .shb, .shs, .theme, .tmp, .udl, .url, .vb, .vbe, .vbp, .vbs, .vhd, .vhdx, .vsmacros, .vsw, .webpnp, .website, .ws, .wsb, .wsc, .wsf, .wsh, .xbap, .xll, .xnk

Resolution:

Change the Outlook on the web mailbox policy to include and exclude the file types that you want.

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:

PS C:\>Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedFileTypes @{Remove = ".xml"}
PS C:\>Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -AllowedFileTypes @{Add = ".xml"}
PS C:\>Get-OwaMailboxPolicy | Set-OwaMailboxPolicy -BlockedMimeTypes @{Remove = "text/xml", "application/xml"}
PS C:\>Get-OwaMailboxPolicy | Set-OwaMailboxPolicy –AllowedMimeTypes @{Add = "text/xml", "application/xml"}

Things to consider:

It may take several minutes before changes to the Outlook on the web policy take effect.

Be aware that by changing the Outlook on the web 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.

Source: Microsoft 365 users can't open or view attachments in Outlook on the web