Update the secret creation forms to handle decoded base64 data from Secrets correctly. To ensure proper handling, we should decrypt the base64 data and determine the MIME type "just in time".

Our current implementation can introduce "mojibake" (the corruption of data caused by incorrect encoding) with certain types of data. This is because we attempt to decode the data into a UTF-16 encoded JavaScript variable, which may not match the encoding of the original data. We need to use an appropriate data structure, like a JS Buffer or Blob, to store the contents of Secret data until we determine its encoding. We can use a tool like file-type to determine the MIME type and then handle it accordingly.