A 25 MB PNG test file

About this file

A 25 MB test file in PNG format, sized to exactly 26,214,400 bytes. Free to download with no signup, for verifying upload limits and file-handling behaviour.

The MD5, SHA-1 and SHA-256 values listed above are the real digests of this file. Recomputing them after download confirms the transfer was not corrupted, and they double as known-good expected values when testing a hashing library.

What this size means in practice

25 MB is widely known as the Gmail attachment limit. In practice, though, mail attachments are Base64-encoded and grow about 33%, so a 25 MB file becomes roughly 33 MB on the wire and usually will not send as-is.

That gap between the advertised limit and the post-encoding size is a pitfall every mail-sending feature eventually hits. Use this file to check whether your size validation runs before or after encoding.

Getting a feel for this size

On a 10 Mbps link the transfer takes roughly 21 seconds, and about 2.1 seconds at 100 Mbps. Base64-encoded it grows to roughly 34,133 KB.

Estimating a photographic PNG at roughly 2 bytes per pixel, this size works out to around 13,107,200 pixels — about 4,180 × 3,135 px at a 4:3 aspect ratio. Compression varies enormously with image content, so treat this purely as an order-of-magnitude figure.

What to check for this format

PNG is lossless and carries an alpha channel. Every file starts with the fixed 8-byte signature 89 50 4E 47 0D 0A 1A 0A, followed by IHDR / IDAT / IEND chunks. The key question for upload validation is whether your code identifies the file by those leading bytes rather than by its extension.

• Whether the alpha channel survives resize and conversion — converting to JPEG destroys it
• Whether image libraries such as GD, ImageMagick, or sharp hit their memory ceiling — PNG memory use scales with decoded pixel count, not file size
• Whether metadata such as tEXt chunks is stripped — PNG has no EXIF, but text chunks can retain information