Helps resolve #676 (specifically the error code issues they identified)
Additionally makes some minor changes to the /usage/import.php form for accessibility and descriptiveness. A more substantial pass will need to be taken (especially for the accessibility of it all) but it's a start.
Rewrites error codes to be accurate and descriptive, splitting out PHP file upload errors from validation errors.
Ensures that files uploaded keep their extension (previously file.tsv would be saved as filetsv in counterstore/, unless added by Sushi)
Rewrites usage\import.php instructions to describe form expectations in greater detail.
Updates link to COUNTER's Code of Practice page.
Revises MAX_FILE_SIZE to use binary bytes.
Adds file type validation to file upload.
Adds labels to form inputs.
Makes the form instructions a figure with figcaption (might not be the best idea, recommended by a Stack Overflow answer)
Helps resolve #676 (specifically the error code issues they identified) Additionally makes some minor changes to the /usage/import.php form for accessibility and descriptiveness. A more substantial pass will need to be taken (especially for the accessibility of it all) but it's a start.