When transferring files from a Mac to a MassTransit server using Firefox's built-in HTTP file transfer support (without the MassTransit Plug-in), you will be able to select two file types that cannot actually be transferred using HTTP. The affected types (.app and .pkg files) are considered "bundles" or "packages" on Mac OS X. They normally look like a single file but are essentially folders containing multiple related files. The HTTP protocol is not designed to transfer folders and is unable to actually transfer these files, but a bug in Firefox allows them to be selected nevertheless.
If you need to transfer .app or .pkg files, you can enclose them in .zip file or install the MassTransit Web Client Plug-in, which allows you to transfer folders, including bundles. You can also avoid the specific bug by using Safari, which will properly prevent selection of bundles.