Microsoft Forms file upload silently writes 0-byte files for guest (B2B) respondents — recreating the question does not help

Jonathan Camilo Cubides 20 Reputation points
2026-09-09T16:12:28.7166667+00:00

Setup

  • A group form (owned by a Microsoft 365 group), audience set to "Only people in my organization can respond".
  • The form has file upload questions. Uploads therefore land in the group site's document library, under Documents > Apps > Microsoft Forms > <form> > <question>.
  • Most of the people who need to answer the form are guest (B2B) accounts in the tenant, not member accounts.

Symptom

For guest respondents, the upload never completes:

  • The file picker accepts the file, a progress bar appears, then it disappears and no file is attached.
  • No error is shown to the responder in most attempts. Some respondents instead see Access denied. You do not have permission to perform this action or access this resource, and others see a message saying the file has expired.
  • On the SharePoint side, each attempt leaves a 0-byte ~tmp placeholder in the question folder. The file is created; the content is never written.

For a member account of the same tenant, the same file uploads normally and the resulting file has its real size.

Control test

Same physical device, same browser, same file, same form, minutes apart. The only variable changed was the responder account.

| Responder account type | Result |

|---|---|

| Member | Uploads correctly, real file size |

| Guest (B2B) | 0-byte ~tmp, silent failure |

Permission matrix — all four combinations tested

Tested with one guest account, granting permissions directly on the upload folder:

| Question / folder | Guest has no grant | Guest granted Submit Files + Edit |

|---|---|---|

| Existing upload question | "Access denied…" | Upload starts, 0-byte file |

| Brand-new upload question (new folder and new link) | n/a | Upload starts, 0-byte file |

Adding the guest to the Forms sharing-link group (Submit Files) removes the "Access denied" message but the content is still never written. Granting full Edit directly on the folder changes nothing.

Already ruled out, with the observed values

| Check | Observed | Verdict |

|---|---|---|

| Require documents to be checked out on the library | No | Not the cause |

| organizationLinkAbilities.canGetSubmitOnlyLink | enabled = true | Organization submit-only links allowed |

| linkExpiration.maxExpirationInDays | 0 | No forced link expiry |

| organizationLinkAbilities.canHaveExternalUsers | enabled = false | Relevant context |

| Form audience | IsAnonymous: false, PermissionForResponder: 1 | Correct for file upload |

| Unmanaged-device / conditional access | Member account succeeds on the same device and browser | Not the cause |

| Require content approval on the library | No | Not the cause |

| Recreate the file upload questions (the documented remediation) | Performed. A brand-new question, with a brand-new folder and a brand-new sharing link, produced the same 0-byte result | Does not resolve |

A second file upload question on the same form, same site and same library has never produced a file with content — not even for the member/owner account. Every file in that question's folder is 0 bytes. The first upload question does produce complete files for the member account.

Questions

  1. Why does the content write fail silently for guest respondents and leave a 0-byte placeholder, instead of surfacing an error? This is the part that worries me most: the failure is invisible to the responder, to the form owner, and to a Power Automate flow consuming the files — Create file with empty content returns Succeeded, so nothing in the run history shows a problem.
  2. Are guest (B2B) respondents supported at all for file upload questions on group forms? Several answers here state it is "by design not supported", but the product still renders the upload control and accepts the attempt rather than hiding or blocking it.
  3. Why does the documented remediation (recreate the upload questions) not work in this case?
  4. If it is unsupported by design, is there a supported alternative for collecting a file from guest respondents while keeping the storage in SharePoint (OneDrive is not an option under our policy)?

Happy to provide tenant-specific identifiers privately if someone from the product team needs them.

Update (after the first answer): confirmed that the File Request alternative is not

available in this environment. Reading the sharing abilities of the upload folder:

anonymousLinkAbilities.canGetSubmitOnlyLink -> enabled = false  (reason 45)

anyoneLinkAbilities.canGetSubmitOnlyLink    -> enabled = false  (reason 45)

anyoneLinkAbilities.canGetEditLink          -> enabled = false  (reason 1)

Per the Enable File Requests documentation, disabling "Anyone" links also disables Request

files — so that path is closed here. File Request also relies on anonymous links, whereas

our respondents are authenticated guest (B2B) accounts.

Also confirmed: a Power Automate flow consuming these uploads reports Succeeded on the

0-byte files, so the failure is invisible in the run history. We have added an explicit

file-size check to the flow.

Microsoft 365 and Office | Microsoft Forms | For business
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  1. Elena-VH 1,490 Reputation points Independent Advisor
    2026-09-09T16:55:58.9566667+00:00

    Dear Jonathan Camilo Cubides,

    Thank you for the detailed troubleshooting and for documenting the testing results so thoroughly.

    1. Why does the content write fail silently and leave a 0-byte placeholder?

    The 0-byte ~tmp file suggests the file was created successfully, but the content upload did not complete. Because the file object exists in SharePoint, processes such as Power Automate may still show the action as successful even though the file contains no data.

    2. Are guest (B2B) respondents supported for file upload questions on group forms?

    According to my research, Microsoft Forms file upload questions are only supported when the form is restricted to people within the organization. Guest or external users are not currently supported for uploading files through Forms, even when SharePoint permissions are manually granted to the target location. For reference: Allowing Guest users to upload files to SharePoint via Group-owned Form

    3. Why does recreating the upload question not resolve the problem?

    Recreating file upload questions is generally intended for situations where the upload folder mapping, sharing link, or SharePoint association has become invalid. However, file upload questions may only need to be recreated after certain SharePoint changes. For reference: Add questions that allow for file uploads in Microsoft Forms

    However, since the separate upload question also produces 0-byte files even for member accounts, it may require access to backend diagnostic logs that are only available to Microsoft Support.

    Therefore, I recommend contacting your Microsoft 365 administrator and asking them to create a support ticket with Microsoft Support for further investigation and troubleshooting. The support team can review tenant-level and service-side logs to determine why the upload process is failing after the temporary file is created and whether this behavior is related to a service limitation, configuration issue, or a backend defect. For reference: Get support for Microsoft 365 for business

    4. Is there a supported alternative for collecting files from guest respondents while storing files in SharePoint?

    If the requirement is to collect files from guest or external users and keep storage within Microsoft 365, you can consider creating a File Request (upload request) using OneDrive or SharePoint, then paste that upload link into the form as a regular question/instruction (e.g., “Please upload your document using this link”). For more information and instructions, please follow this article: Create a file request - Microsoft Support 

    Note:

    I hope the information shared is helpful. If you have any questions or if I have misunderstood anything, please feel free to reach out. I'm looking forward to your update.

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