Fixed

With more than one FileSender account in Thunderbird, removing an attachment uploaded
with one account could delete the upload of another account's attachment, whose link
then stopped working. Aborting an upload or answering "reuse the link?" could also
reach the other account's file. The uploads of each account are now kept apart.
A base URL written with an upper-case scheme (HTTPS://...) made every request fail
as "credentials rejected".
The 16 px icon now shows the envelope like the other sizes.

Added

The account settings warn when the base URL differs from the address the FileSender
server declares (for example a host name alias): its links are on that address and
would not be reused.

Security

A previous link is reused only if it is under the account's base URL, so the download
token of another server's link is never sent to the configured server (for example
after changing the base URL of an account).
Requests to FileSender no longer follow redirects, which could send the file content
to another address.
Upload errors show only the FileSender error code, never free text from the server or
a proxy.
The instance configuration is never downloaded over plain http://.
Download links without an http(s) scheme are ignored.