Selecting five files in a dialog is only one stage of an upload. A form can display all five names while transfer, validation or saving is still incomplete.
The HTML multiple attribute allows applicable controls to accept more than one value. It does not promise that an application will complete every subsequent operation successfully.
Look for a separate status for each file when the site provides one. A final summary should distinguish saved items from rejected or interrupted ones. Do not submit the same group repeatedly just because a single progress indicator is unclear; first check the existing result to avoid duplicates.
For a test form you control, try two small permitted files and one harmless unsupported file. Verify that the interface explains which succeeded and what action remains. The test should not require private documents.
If replacing a selection, check whether the control adds to the list or substitutes a new list. Custom interfaces can behave differently, so read the visible result before sending.
A useful support message names the stage: selected, transferring, rejected or saved but unavailable. That sequence turns “the upload failed” into a problem someone can investigate without guessing whether the files ever left the selection dialog.
Illustration selected from this site’s existing collection.