API acceptance tests - do not only check hard-coded mime type

[individual-it]

in Sharing.php several methods check the mime type of a downloaded file checkLastPublicSharedFileDownload() , checkLastPublicSharedFileWithPasswordDownload() ,theUserShouldBeAbleToDownloadTheFileUsingTheSharingApi()
but "text/plain" is hardcoded as mime-type
make that more flexible to be able to check also other mime-types

[ownclouders]

GitMate.io thinks possibly related issues are #22492 (test), #22485 (test), #31579 (test), #32040 (fix acceptance tests), and #31516 (Acceptance test coding standard changes).