`` limit selectable files by extensions

Deeper dive into file type restrictions

Fine-tuning with the accept attribute

Although the accept attribute is useful for limiting file types, bear in mind its behavior varies amongst browsers. For instance, accept attribute support commenced from Microsoft Internet Explorer 9, so always conduct browser compatibility checks for your target audience.

Client-side validation with JavaScript

Besides the simplicity of HTML5 solutions such as the accept attribute, incorporating JavaScript checks can offer a more immediate user response for incorrect file types:

document.getElementById("myFileInput").addEventListener('change', function(event) {
  // "You shall not pass!" - Gandalf the Grey on handling invalid file types
  var validExtensions = ['jpg', 'png', 'gif']; // list of valid extensions
  var fileName = event.target.files[0].name;
  var fileExtension = fileName.split('.').pop().toLowerCase();
  if(validExtensions.indexOf(fileExtension) < 0) {
    alert("Invalid file type, you cheeky imposter!");
    this.value = ''; // Resetting the input value to nought
  }
});
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Fortifying with server-side checks

Regardless of client-side precautions, never place complete trust in user inputs. Always enforce server-side validation of file size and type to safeguard your application from harmful uploads. Implementing such robust server-side checks enhances system security and preserves system integrity.

Cross-browser support and alternatives

Consistency across browsers

While the accept attribute is handy, its behavior differs across browsers. For instance, Google Chrome and Mozilla Firefox may handle file filtering differently compared to Safari or older versions of IE. It's crucial to carry out extensive testing to account for these discrepancies.

Alternative methods for legacy browsers

For browsers that do not support the accept attribute, or in cases where JavaScript is disabled, have a backup plan such as Flash or Java-based uploaders. Aim for a graceful degradation in user experience that doesn't make the user with older technology feel left out.

Harnessing client-server synergy

Enhancing UX with XMLHttpRequest

For an enriching user experience, consider using XMLHttpRequest or the Fetch API to handle file uploads. These allow you to listen to upload events, providing real-time feedback about the progress and completion of file uploads:

var xhr = new XMLHttpRequest(); 
xhr.open('POST', 'upload_endpoint'); 
xhr.upload.onprogress = function(event) { 
  if (event.lengthComputable) { 
    var percentComplete = (event.loaded / event.total) * 100; 
    console.log('Upload progress: ' + percentComplete + '%, faster than a caffeinated coder!');
  } 
}; 
xhr.onload = function() { 
  if (xhr.status == 200) { 
    console.log('Upload victory: ' + xhr.responseText); 
  } else { 
    console.log('Upload hiccup: ' + xhr.statusText); 
  } 
}; 
xhr.send(new FormData(document.getElementById('uploadForm'))); 
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Solidifying with server-side logic

Regardless of how refined your client-side validation is, be ready for scenarios where a wrong file type may sneak through. Always implement server-side logic that checks the complete file name and validates its actual extension, as the last line of defense.