readAsDataURL should be more precise about the data URL contents #104

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1. annevk commented on Jun 21, 2018

   

   annevk

   on Jun 21, 2018

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   See also whatwg/fetch#665 which this would tie into (if someone wants to work on that, feel free, happy to assist).
2. inexorabletash commented on Jun 21, 2018

   

   inexorabletash

   on Jun 21, 2018

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   Re: "creating the string from the contents of the type attribute" - just a note in case someone tackles this. Beware of the gotcha associated with #43 - namely, as the spec is currently written, the value returned by the type attribute's getter and the internal type of the blob (which isn't clearly documented) differ if the type is not a parseable MIME type.
3. shannonbooth commented on Apr 8, 2026

   

   shannonbooth

   on Apr 8, 2026

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   Regarding the mime type, WPT tests that an unspecified mime type should be application/octet-stream which all browser implementations have aligned on.

   https://github.com/web-platform-tests/wpt/blob/844e3e5ec7d746c04c8aabb5ab5e8e3732ce732c/FileAPI/reading-data-section/filereader_readAsDataURL.any.js#L42
4. shannonbooth commented on Apr 8, 2026

   

   shannonbooth

   on Apr 8, 2026

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   Though, saying that - it's not clear whether the mime type should fall back to that as well for a mime type which is parsable or not.

   Based on the following test, I think the logic is probably just if it is not an empty string in all browsers at least:

   async_test(function(testCase) {
     var blob = new Blob(["TEST"], { type: 'not a mime type' });
     var reader = new FileReader();
   
     reader.onload = this.step_func(function() {
       assert_equals(reader.result,
                     "data:not a mime type;base64,VEVTVA==");
       testCase.done();
     });
     reader.readAsDataURL(blob);
   }, 'readAsDataURL result for Blob with unparsable MIME type');
5. annevk commented on Apr 9, 2026

   

   annevk

   on Apr 9, 2026

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   There's a separate issue on making sure type is always valid. We should really attempt to fix that.

   👍React with 👍1Reacted by Shannon Booth