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Having problems opening an ATTACHMENT file?

Learn about files with extension ATTACHMENT

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ATTACHMENT File

What are ATTACHMENT files and how to open them

Are you having problems opening an ATTACHMENT file or are you simply curious about its contents? We're here to explain the properties of these files and provide you with software that can open or handle your ATTACHMENT files.

What is an ATTACHMENT file?

An .ATTACHMENT file is a 1Password Cloud Keychain file.

1Password Cloud Keychain files store file attachments uploaded into the user vault of 1Password, saved separately from the textual data like passwords or notes. Each file attachment is encrypted and managed alongside other 1Password data, but kept in its own binary blob so that large files or non-text documents do not bloat the main JSON or record database. These attachments can be things like PDFs, images, or other user-supplied files held as part of a 1Password item.

These files are created by the 1Password system itself when a user adds a file attachment to an item. They are read or decrypted by 1Password whenever the associated item is accessed and attachments are requested. The files are not meant to be edited directly; they are opaque encrypted containers handled automatically by 1Password's syncing, backup, and SDK infrastructure.

Files of this kind begin with the signature OPCLDAT in ASCII at offset zero (bytes 0x4F50434C444154) as an internal identifier.

How to open ATTACHMENT files

Important: Different programs may use files with the ATTACHMENT file extension for different purposes, so unless you are sure which format your ATTACHMENT file is, you may need to try a few different programs.

We have not yet verified any programs for Windows that work with this specific file format. If you know one, please use the 'Suggest a program' link below. Thanks!

Last updated: August 22, 2026

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