CSS crop string in the middle

This is the best I can come up with... It might be worthwhile trying to clean up the leading edge of the second span...

CSS

#fileName span {
      white-space: nowrap;
      overflow: hidden;
      display:inline-block;
 }
#fileName span:first-child {
      width: 100px;
      text-overflow: ellipsis;
 }
#fileName span + span {
      width: 30px;
      direction:rtl;
      text-align:right;
 }

HTML

<div id="fileName">
    <span>This is the big name of my file.txt</span>
    <span>This is the big name of my file.txt</span>
</div>

http://jsfiddle.net/c8everqm/1/

Here is a clean CSS solution using the data-* attribute and two ::after pseudo-elements. I also added an optional hover and show all text (the #fileName::after pseudo element needs to be removed when the full text is shown).

Example 1

#fileName {
  position: relative;
  width: 100px;
}

#fileName p {
  white-space: nowrap;
  text-overflow: ellipsis;
  overflow: hidden;
}

#fileName:after {
  content: attr(data-filetype);
  position: absolute;
  left: 100%;
  top: 0;
}


/*Show on hover*/

#fileName:hover {
  width: auto
}

#fileName:hover:after {
  display: none;
}

<div id="fileName" data-filetype="txt">
  <p>This is the big name of my file.txt</p>
</div>

Going further — hiding the appended filetype when the filename is short

The #fileName p::after is given a background color that matches the background of the text. This covers the ".txt" when the filenames are short and therefore not cut off with overflow: hidden.

Note the padding-right: 22px, this pushes the ".txt" beyond the ellipsis.

Refer to examples 2 and 3 below for different methods with different browser support for each. It doesn't seem to be possible to hide the ".txt" happily in all browsers.

Example 2

Browser Compatibility: Chrome and Firefox.

The #fileName p::after is given a background color that matches the background of the text. This covers the ".txt" when the filenames are short and therefore not cut off with overflow: hidden.

Note the padding-right on each of the ::after pseudo-elements. padding-right: 22px pushes the ".txt" beyond the ellipsis and padding-right: 100% gives the covering pseudo-element its width. The padding-right: 100% doesn't work with Edge or IE 11.

#fileName {
  position: relative;
  width: 122px;
}

#fileName::after {
  content: attr(data-filetype);
  position: absolute;
  right: 0;
  top: 0;
}

#fileName p {
  white-space: nowrap;
  text-overflow: ellipsis;
  overflow: hidden;
  padding-right: 22px;
}

#fileName p::after {
  content: '';
  background: #FFF;
  position: relative;
  padding-right: 100%;
  z-index: 1;
}

/*Show on hover*/

#fileName:hover {
  width: auto;
}


/*Hide .txt on hover*/

#fileName:hover::after {
  display: none;
}

<div id="fileName" data-filetype=".txt">
  <p>This is the big name of my file.txt</p>
</div>

<div id="fileName" data-filetype=".txt">
  <p>Short.txt</p>
</div>

Example 3

Browser Compatibility: IE 11, Edge and Chrome.

The content: ... unholy amount of ... on #fileName p::after gives it width. This, along with display: inline-block, is currently the only method that works on the Edge browser / IE 11 as well as Chrome. The display: inline-block breaks this method on Firefox and the .txt is not covered on short filenames.

#fileName {
  position: relative;
  width: 122px;
}

#fileName::after {
  content: attr(data-filetype);
  position: absolute;
  right: 0;
  top: 0;
  padding-right: 10px; /*Fixes Edge Browser*/
}

#fileName p {
  white-space: nowrap;
  overflow: hidden;
  padding-right: 22px;
  text-overflow: ellipsis;
}

#fileName p::after {
  content: '.........................................................................................................................';/*Fixes Edge Browser*/
  background: #FFF;
  position: relative;
  display: inline-block;/*Fixes Edge Browser*/
  z-index: 1;
  color: #FFF;
}


/*Show on hover*/

#fileName:hover {
  width: auto
}

#fileName:hover::after {
  display: none;
}

<div id="fileName" data-filetype=".txt">
  <p>This is the big name of my file.txt</p>
</div>

<div id="fileName" data-filetype=".txt">
  <p>Short.txt</p>
</div>

Here is another suggestion that worked well for me:

<div style="width:100%;border:1px solid green;display:inline-flex;flex-wrap:nowrap;">
   <div style="flex: 0 1 content;text-overflow: ellipsis;overflow:hidden;white-space:nowrap;"> Her comes very very very very very very very very very very very very very very very very very very very long </div>
   <div style="flex: 1 0 content;white-space:nowrap;"> &nbsp;but flexible line</div>
</div>