The upload that keeps failing for no obvious reason
You try to attach a screenshot or scan to an email or upload form, and it gets rejected for being too large — except it's just one image, and it looks completely ordinary. Check the file extension and it's often .bmp: a format that stores every pixel's color value with little to no compression, so a screenshot that would be 300KB as a JPG can easily be 10-15MB as a BMP.
This isn't a broken or corrupted file — BMP was simply designed decades ago, before efficient compression was standard, and it still shows up today from older Windows tools, legacy scanning software, and some specialized hardware that defaults to it.
Where BMP files actually come from today
Nobody deliberately chooses BMP anymore for sharing — it shows up as a side effect. Older Windows utilities (Paint's older defaults, some legacy screenshot tools), industrial or lab equipment that exports imaging data in BMP, and old scanning software from a decade-plus ago are the most common sources. If you've inherited an old archive of scanned documents or images, there's a good chance a chunk of it is sitting in BMP without anyone having chosen that deliberately.
The fix is almost always the same: convert to JPG before doing anything else with the file, since virtually nothing modern expects to receive BMP directly.
How much smaller does it actually get
The size reduction from BMP to JPG is dramatic precisely because BMP starts from near-zero compression — converting typically shrinks the file by 80-95%, sometimes more, depending on the image content. A 14MB BMP screenshot commonly becomes well under 1MB as a JPG at a high quality setting, with no visible difference to the eye.
This isn't a case of choosing between quality and size on a fine margin — BMP's overhead is so large that even a conservative, high-quality JPG setting captures nearly all the savings available.
One thing that usually isn't a concern here
Converting from PNG or WebP to JPG means worrying about transparent areas turning into a solid background color, since JPG has no transparency support. BMP mostly sidesteps this — the vast majority of real-world BMP files are fully opaque to begin with, since transparency in BMP is technically possible but almost never actually used. That makes a BMP-to-JPG conversion one of the more predictable, low-surprise format conversions you'll run into.