fix: show the correct error for a single folder attachment

[chrispader]

Dragging a single folder into the attachment picker currently shows the generic corrupted-file error even though the app identifies it as a folder. The folder-specific message is only selected when the validation state marks the selection as multiple files. This PR uses the folder-specific error for both single- and multi-item selections and adds focused regression coverage.

Explanation of Change

This originated while re-evaluating the unfinished attachment-validation work in #85751. Most of that work was later covered by #86189, which corrected receipt attachment validation, #93234, which made multi-file validation sequential and cleaned up object URLs, and #92236, which expanded useFilesValidation coverage. Related fixes in #96024 and #96940 also addressed HEIC conversion failures and file snapshot reliability. Comparing those changes with the current implementation exposed one remaining gap: getFileValidationErrorText() only mapped FOLDER_NOT_ALLOWED to the folder-specific copy during multi-file validation.

This change moves that mapping into the common error switch so the result no longer depends on the number of selected items. Unit tests cover both a single folder and a folder in a multi-item selection.

Fixed Issues

$ #100707
PROPOSAL: N/A

Tests

1. Sign in on desktop web and open any report.
2. Drag a single folder into the report composer.
3. Verify an Attachment error modal appears with Uploading a folder is not allowed. Please try a different file.
4. Verify the generic corrupted-file message does not appear.
5. Drag a folder and a valid file into the report composer together.
6. Verify the same folder-specific error appears.

• Verify that no errors appear in the JS console

Offline tests

1. Go offline.
2. Repeat steps 1–4 from Tests.
3. Verify the same folder-specific error appears.

QA Steps

Same as Tests.

• Verify that no errors appear in the JS console

PR Author Checklist

• I linked the correct issue in the ### Fixed Issues section above
• I wrote clear testing steps that cover the changes made in this PR
  • I added steps for local testing in the Tests section
  • I added steps for the expected offline behavior in the Offline steps section
  • I added steps for Staging and/or Production testing in the QA steps section
  • I added steps to cover failure scenarios (i.e. verify an input displays the correct error message if the entered data is not correct)
  • I turned off my network connection and tested it while offline to ensure it matches the expected behavior (i.e. verify the default avatar icon is displayed if app is offline)
  • I tested this PR with a High Traffic account against the staging or production API to ensure there are no regressions (e.g. long loading states that impact usability).
• I included screenshots or videos for tests on all platforms
• I ran the tests on all platforms & verified they passed on:
  • Android: Native
  • Android: mWeb Chrome
  • iOS: Native
  • iOS: mWeb Safari
  • MacOS: Chrome / Safari
• I verified there are no console errors (if there's a console error not related to the PR, report it or open an issue for it to be fixed)
• I followed proper code patterns (see Reviewing the code)
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• I followed the guidelines as stated in the Review Guidelines
• I tested other components that can be impacted by my changes (i.e. if the PR modifies a shared library or component like Avatar, I verified the components using Avatar are working as expected)
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  • A similar style doesn't already exist
  • The style can't be created with an existing StyleUtils function (i.e. StyleUtils.getBackgroundAndBorderStyle(theme.componentBG))
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  • The assets load correctly across all supported platforms.
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• I added unit tests for any new feature or bug fix in this PR to help automatically prevent regressions in this user flow.
• If the main branch was merged into this PR after a review, I tested again and verified the outcome was still expected according to the Test steps.

Screenshots/Videos

Pending manual verification.

Android: Native

Android: mWeb Chrome

iOS: Native

iOS: mWeb Safari

MacOS: Chrome / Safari

[chrispader]

@MelvinBot create a GH issue for this PR and assign me to it. Update the PR description to link to this issue.

[MelvinBot]

Done — created #100707 and assigned chrispader.

Issue: Single folder attachment shows the generic corrupted-file error instead of the folder-specific error

I also updated this PR description:

• ### Fixed Issues now links the new issue.
• Checked the "I linked the correct issue" box in the author checklist.

I searched both Expensify/App and Expensify/Expensify for open and recently closed duplicates and found none.

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[melvin-bot]

@ikevin127 Please copy/paste the Reviewer Checklist from here into a new comment on this PR and complete it. If you have the K2 extension, you can simply click: [this button]

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-09-09T12:40:29.880401Z	3aa1f54	Draft marked ready

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