Issue with mimetype when sending files from other apps

[gagelarsen]

• I have searched open and closed issues for duplicates
• I am submitting a bug report for existing functionality that does not work as intended
• This isn't a feature request or a discussion topic

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Bug description

When sending files from another app (BGStats) to other signal users it seems that they are unable to open the sent file with the correct app. This seems to be caused because the attachments mime type is not being set correctly by signal. I have tested this by sending a file from BGStats to another user using the basic iOS messaging app and they can open the file just fine that way. This also doesn't seem to be a problem for android users. They can send BGStat files using signal with now problems. Rather than seeing the file as an *.bgstats file it sees it as a macbinary archive.

Steps to reproduce

• Share a file from the app "BGStats" - (I presume other apps using similar file types, JSON, would do the same thing.)
• Select Signal as the app you want to use to share
• Select your group or person you want to send it to.

Actual result: When they select the file and hit the share button the BGStats app does not appear as one of the applicable apps to use.

Expected result: After selecting share you should be able to select BGStats in the applicable app section.

Screenshots

When shared via signal:

When shared via text:

Device info

Device: iPhone 15 Pro Max

iOS version: 18.2.1

Signal version: 7.42

Link to debug log

https://debuglogs.org/ios/7.41.3/beec624aa51e0dddca7fe2e6b2b7456812aea064d1897ca89b3d98f4736b970a.zip