Today I needed to link to a local file on a network share from a page in TYPO3. This link did not start with the usual http:// but instead with file:///. The problem was, that the rich text editor (RTE) in the backend of TYPO3 always prepends „invalid“ links with http:// and I ended up with http://file:///path-to-network-share, which of course did not work correctly.
I tried editing the RTE parse function in TYPOScript but that did not solve my problem. So, after a while of searching the internet I found the answer: append rtekeep="1" to the link like in the following example. This tells the RTE to leave the link untouched.
<a href="file:///path-to-network-share" rtekeep="1">Link to file</a>