Resolve "'Please select a file' when attempting to upload or replace from the UI"

What does this MR do?

• Disable Submit button while uploading file
• Hide file upload placeholder text if file exists
• vertically center place text

Are there points in the code the reviewer needs to double check?

• Should we show an overlay while uploading to make it clearer than just the progress bar?
• this should only affect file upload/replace on view blob page. Anywhere else is not intentional :)

Why was this MR needed?

Screenshots (if relevant)

upload-file

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

Closes #35052 (closed)