Skip file type guess or lock accepted file type

The file type guess [seen here](https://github.com/getodk/pyxform-http/blob/82825166d0ec9c5fd0ea577da868f15bb6285e42/app/main.py#L35) could be skipped since pyxform will try parsing as XLSX first and then try XLS (then concatenated formats - markdown then csv) until one of them parses OK. If a file type is specified to `xls2xform.convert` (see https://github.com/getodk/pyxform-http/issues/73) then only that type is attempted. Perhaps only accepting XLSX at xlsform online https://getodk.org/xlsform/ could be a way to wean off XLS and therefore allow pyxform to drop the sporadically maintained dependency `xlrd`.

Start with app/main.py around the linked file-type guess and read issue #73 for the proposed xls2xform.convert type parameter. Determine whether the accepted behavior should skip guessing or restrict uploads to XLSX. Done means the selected file-type behavior is implemented and XLS/XLSX parsing is covered by the repository's existing checks.

Written by the indexing model from the issue text.