fix: correctly validate file URIs with streams

Wolfgang Zieglerrequested to merge
issue/canvas-3591986:3591986-canvas-breaks-uploading-files-with-spaces-for-the-whole-site into 1.x

Changes how file URIs are validated: handles URIs with streamwrappers correctly

Testing instructions

  • create a file upload field
  • upload a file with a filename containing spaces

Closes #3591986

Edited by Wolfgang Ziegler

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